media: platform: cros-ec-cec: Fix dependency on MFD_CROS_EC

Without the MFD driver, we run into a link error:

drivers/media/platform/cros-ec-cec/cros-ec-cec.o: In function `cros_ec_cec_transmit':
cros-ec-cec.c:(.text+0x474): undefined reference to `cros_ec_cmd_xfer_status'
drivers/media/platform/cros-ec-cec/cros-ec-cec.o: In function `cros_ec_cec_set_log_addr':
cros-ec-cec.c:(.text+0x60b): undefined reference to `cros_ec_cmd_xfer_status'
drivers/media/platform/cros-ec-cec/cros-ec-cec.o: In function `cros_ec_cec_adap_enable':
cros-ec-cec.c:(.text+0x77d): undefined reference to `cros_ec_cmd_xfer_status'

As we can compile-test all the dependency, the extra '| COMPILE_TEST' is
not needed to get the build coverage, and we can simply turn MFD_CROS_EC
into a hard dependency to make it build in all configurations.

Fixes: cd70de2d ("media: platform: Add ChromeOS EC CEC driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>