HID: google: add dependency on Cros EC for Hammer

Whiskers tablet mode support needs access to Chrome Embedded Controller,
so we need to add dependency on MFD_CROS_EC.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: eb1aac4c ("HID: google: add support tablet mode switch for Whiskers")
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>