media: rc: ir-nec-decoder: move scancode composing code into a shared function

The NEC scancode composing and protocol type detection in
ir_nec_decode() is generic enough to be a shared function.  Let's create
an inline function in rc-core.h, so that other remote control drivers
can reuse this function to save some code.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>