Input: cros_ec_keyb - replace KEYBOARD_CROS_EC dependency

The ChromeOS EC keyboard driver config depends on CROS_EC_PROTO but
MFD_CROS_EC selects CROS_EC_PROTO instead. Mixing select and depends
on is bad practice as it may lead to circular Kconfig dependencies.

Since the platform device that is matched with the keyboard driver
is registered by the ChromeOS EC mfd driver, KEYBOARD_CROS_EC really
should depend on MFD_CROS_EC. And because this config option selects
CROS_EC_PROTO, that dependency is met as well. So make the driver
to depend on MFD_CROS_EC instead of CROS_EC_PROTO.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>