V4L/DVB (13540): ir-common: Cleanup get key evdev code

V4L/DVB (13540): ir-common: Cleanup get key evdev code

The same loop to seek for a key were used on different places. Also,
no spinlock were protecting it to avoid the risk of replacing a keycode
while seeking for a new code.

This cleanup does:
	- create an unique function to seek for a code;
	- adds an spinlock to protect the table lookup;
	- remove some unused code;
	- simplifies to code to make it easier to understand.

Basically no change in behavior should be noticed after this patch.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>