ACPI / EC: Cleanup transaction state transition.

This patch collects transaction state transition code into one function. We
then could have a single function to maintain transaction transition
related behaviors. No functional changes.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Tested-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Tested-by: Tigran Gabrielyan <tigrangab@gmail.com>
Tested-by: Adrien D <ghbdtn@openmailbox.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>