platform/chrome: cros_ec_proto: drop BUG_ON() in cros_ec_get_host_event()

It is overkill to crash the kernel if the `ec_dev` doesn't support MKBP
event but gets called into cros_ec_get_host_event().

Drop the BUG_ON() and return error (0 in the case) instead.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220513044143.1045728-5-tzungbi@kernel.org