mfd: cros_ec_dev: Return number of bytes read with CROS_EC_DEV_IOCRDMEM

While ioctls normally return a negative error or 0 on success, this is not
the case for CROS_EC_DEV_IOCXCMD in the cros_ec driver, which returns the
number of bytes read instead. However, CROS_EC_DEV_IOCRDMEM in the same
driver does not return the number of byte read on success. This is both
inconsistent and problematic, since the amount of returned data can be
dynamic. Have it return the number of data bytes for consistency.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>