i2c: Fix the i2c_smbus_read_i2c_block_data() prototype

Let the drivers specify how many bytes they want to read with
i2c_smbus_read_i2c_block_data(). So far, the block count was
hard-coded to I2C_SMBUS_BLOCK_MAX (32), which did not make much sense.
Many driver authors complained about this before, and I believe it's
about time to fix it. Right now, authors have to do technically stupid
things, such as individual byte reads or full-fledged I2C messaging,
to work around the problem. We do not want to encourage that.

I even found that some bus drivers (e.g. i2c-amd8111) already
implemented I2C block read the "right" way, that is, they didn't
follow the old, broken standard. The fact that it was never noticed
before just shows how little i2c_smbus_read_i2c_block_data() was used,
which isn't that surprising given how broken its prototype was so far.

There are some obvious compatiblity considerations:
* This changes the i2c_smbus_read_i2c_block_data() prototype. Users
  outside the kernel tree will notice at compilation time, and will
  have to update their code.
* User-space has access to i2c_smbus_xfer() directly using i2c-dev, so
  the changed expectations would affect tools such as i2cdump. In order
  to preserve binary compatibility, we give I2C_SMBUS_I2C_BLOCK_DATA
  a new numeric value, and define I2C_SMBUS_I2C_BLOCK_BROKEN with the
  old numeric value. When i2c-dev receives a transaction with the
  old value, it can convert it to the new format on the fly.

Signed-off-by: Jean Delvare <khali@linux-fr.org>

Documentation/i2c/chips/max6875

@@ -99,7 +99,7 @@ And then read the data
 
   or
 
-  count = i2c_smbus_read_i2c_block_data(fd, 0x84, buffer);
+  count = i2c_smbus_read_i2c_block_data(fd, 0x84, 16, buffer);
 
 The block read should read 16 bytes.
 0x84 is the block read command.

Documentation/i2c/writing-clients

@@ -571,7 +571,7 @@ SMBus communication
                                         u8 command, u8 length,
                                         u8 *values);
   extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
-                                           u8 command, u8 *values);
+                                           u8 command, u8 length, u8 *values);
 
 These ones were removed in Linux 2.6.10 because they had no users, but could
 be added back later if needed: