V4L/DVB: em28xx: fix a regression caused by the rc-map changes

The patch that adds the rc-map changes didn't take into account that an
a table with IR_TYPE_UNKNOWN would make change_protocol to return -EINVAL.

As this function is fundamental to initialize some data, including a
callback to the getkey function, this caused the driver to stop working,
hanging the machine most of the times.

The fix were simply to add a handler for the IR type, but, to avoid further
issues, explicitly call change_protocol and handle the error before
initializing the IR. Also, let input_dev to start/stop IR handling,
after the opening of the input device.

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