mfd: cros_ec: Remove CROS_EC_PROTO dependency for SPI and I2C drivers

The ChromeOS EC SPI and I2C transport drivers depends on CROS_EC_PROTO
but MFD_CROS_EC select CROS_EC_PROTO instead. Mixing select and depends
on is bad practice as it may lead to circular Kconfig dependencies.

Since these drivers already depend on MFD_CROS_EC and that config option
already selects CROS_EC_PROTO, there is no need to make them explicitly
depend on CROS_EC_PROTO since that dependency is already met.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>