Input: keyboard: Fix ChromeOS EC keyboard help message.

The cros-ec I2C and SPI transport drivers have been moved from MFD
subsystem to platform/chrome, at the same time, the config symbol
has been renamed and lost the MFD_ prefix So, update the help message
accordingly.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Benson Leung <bleung@chromium.org>