mfd: twl-core: Improve the documentation

Saying it "returns the result" seems tautological.  The read function
does not return num_bytes on success, it returns zero on success.  I
noticed this discrepancy because some of the callers were checking for
>= 0.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>