clk: fix clk_get_rate() documentation

Improve clk_get_rate() @return documentation that otherwise is a bit
ambiguous. At the moment I expect to return 0 as error since the return
type is 'ulong', instead the function really returns negative value in
case the corresponding function pointer is null and returns 0 if the clock
is invalid.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>

include/clk.h

@@ -389,7 +389,8 @@ int clk_free(struct clk *clk);
  *
  * @clk:	A clock struct that was previously successfully requested by
  *		clk_request/get_by_*().
- * @return clock rate in Hz, or -ve error code.
+ * @return clock rate in Hz on success, 0 for invalid clock, or -ve error code
+ *	   for other errors.
  */
 ulong clk_get_rate(struct clk *clk);