[media] rc-core: fix dib0700 scancode generation for RC5

commit af3a4a9b ("[media] dib0700: NEC scancode cleanup") cleaned
up the NEC scancode logic but overlooked the RC5 case.

This patch brings the RC5 case in line with the NEC code and makes
the struct self-documenting.

Signed-off-by: David Härdeman <david@hardeman.nu>
Reported-by: David Cimbůrek <david.cimburek@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>