V4L/DVB: IR: only initially registers protocol that matches loaded keymap

Rather than registering all IR protocol decoders as enabled when bringing
up a new device, only enable the IR protocol decoder that matches the
keymap being loaded. Additional decoders can be enabled on the fly by
those that need to, either by twiddling sysfs bits or by using the
ir-keytable util from v4l-utils.

Functional testing done with the mceusb driver, and it behaves as expected,
only the rc6 decoder is enabled, keys are all handled properly, etc.

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