V4L/DVB (9607): em28xx: Properly implement poll support for IR's

The first em28xx were based on i2c IR's. However, some newer designs
are coming with a polling-based IR. Those are done by reading a register
set at em28xx.

This patch adds core polling support for those devices. Later patches will
add support for some device-specific IR's.

This patch adds the same basic IR polling code used by bttv, cx88 and saa7134, and
shares the common getkey masks defined at ir-common.

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