platform/chrome: wilco_ec: event: Check ACPI_COMPANION()

Every platform driver can be forced to match a device that doesn't match
its list of device IDs because of device_match_driver_override(), so
platform drivers that rely on the existence of a device's ACPI companion
object need to verify its presence.

Accordingly, add a requisite ACPI_COMPANION() check against NULL to the
wilco_ec event driver.

Fixes: 27d58498f690 ("platform/chrome: wilco_ec: event: Convert to a platform driver")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/2076666.usQuhbGJ8B@rafael.j.wysocki