V4L/DVB: em28xx-input: Don't generate one debug message for every get_key read

Instead of generating one printk for every IR read, prints it only when
count is different from the last count.

While here, as this code is called on every 100ms during the runtime
lifetime, do some performance optimization, assuming that, under normal
circumstances, it is unlikely that the driver would get a new key/key
repeat on every poll.

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