ACPI: EC: Clean up status flags checks in advance_transaction()

Eliminate comparisons from the status flags checks in
advance_transaction() (especially from the one that is only correct,
because the value of the flag checked in there is 1) and rearrange
the code for more clarity while at it.

No intentional functional impact.

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