i2c: Replace I2C_CROS_EC_TUNNEL dependency

The ChromeOS EC tunnel I2C bus driver depend 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 the platform device that is matched with the I2C bus driver
is registered by the ChromeOS EC mfd driver, I2C_CROS_EC_TUNNEL
really depends 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: Wolfram Sang <wsa@the-dreams.de>