firmware: avoid invalid fallback aborts by using killable wait

Commit <02342e8b12b969d>("firmware: avoid invalid fallback aborts by
using killable swait") switches to killable swait in order to prevent
erroneous signals such as SIGCHLD from causing the kernel to think
that the userspace daemon has unsuccessful in writing the firmware.

Since the above commit changes modified APIs in firmware code, mirror
the same behavior on our older firmware_class version.

Change-Id: Ie06a36bf229e62bb77e473d43bfdc37f3f16c808
Signed-off-by: Kyle Yan <kyan@codeaurora.org>