ACPI: EC: Fold acpi_ec_check_event() into acpi_ec_event_handler()

Because acpi_ec_event_handler() is the only caller of
acpi_ec_check_event() and the separation of these two functions
makes it harder to follow the code flow, fold the latter into the
former (and simplify that code while at it).

No expected functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>