The lirc TX functionality expects the process which writes (TX) data to
the lirc dev to sleep until the actual data has been transmitted by the
hardware.

Since the same timeout calculation is duplicated in more than one driver
(and would have to be duplicated in even more drivers as they gain TX
support), it makes sense to move this timeout calculation to the lirc
layer instead.

At the same time, centralize some of the sanity checks.
Signed-off-by: David Härdeman <david@hardeman.nu>
Cc: Jarod Wilson <jwilson@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>