V4L/DVB: IR: export ir_keyup so imon driver can use it directly

The imon driver currently reimplements its own version of ir_keyup
(along with key release timer functionality also already present in the
core IR code). A follow-up imon patch will make use of ir_keyup and the
IR stack's key release code.

Trivial extraction from David Härdeman's pending rc-core merge and
device interface abstraction patchset to facilitate merging a patch
based on his imon input dev split patch ahead of the larger churn, which
is slated for post-2.6.37-rc1 (after Dmitry's large keycode patches are
merged in mainline).

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