[media] lirc_zilog: Restore checks for existence of the IR_tx object

This reverts commit 8090232a and
adds an additional check for ir->tx == NULL.

The user may need us to handle an RX only unit.  Apparently
there are TV capture units in existence with Rx only wiring
and/or RX only firmware for the on-board Zilog Z8 IR unit.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>