[media] em28xx: add support for RC6 mode 0 on devices that support it

[media] em28xx: add support for RC6 mode 0 on devices that support it

Newer em28xx chipsets (em2874 and upper) are capable of supporting
RC6 codes, on both mode 0 (command mode, 16 bits payload size, similar
to RC5, also called "Philips mode") and mode 6a (OEM command mode,
with offers a few alternatives with regards to the payload size).
I don't have any mode 6a control ATM to test it, so, I opted to add
support only to mode 0.
After this patch, adding support to mode 6a should not be hard.
Tested with a Philips television remote controller.

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