[media] nuvoton-cir: add proper rx fifo overrun handling

Per discussion with Andy Walls on irc, rx fifo overruns are not all that
uncommon on a busy system, and the initial posting of the nuvoton-cir
driver doesn't handle them well enough. With this addition, we'll drain
the hw fifo, attempt to process any ir pulse trains completed with that
flush, then we'll issue a hw rx fifo clear and reset the raw ir sample
kfifo and start over collecting raw ir data.

Also slightly refactors the cir interrupt enabling so that we always get
consistent flags set and only have to modify them in one place, should
they need to be altered.

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