[media] em28xx: add support for NEC proto variants on em2874 and upper

By disabling the NEC parity check, it is possible to handle all 3 NEC
protocol variants (32, 24 or 16 bits).
Change the driver in order to handle all of them.
Unfortunately, em2860/em2863 provide only 16 bits for the IR scancode,
even when NEC parity is disabled. So, this change should affect only
em2874 and newer devices, with provides up to 32 bits for the scancode.
Tested with one NEC-16, one NEC-24 and one RC5 IR.

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