[media] lirc_zilog: Remove unneeded tests for existence of the IR Tx function

The driver is now structured so that it must handle an IR Tx unit
for a Z8 IR chip, or it refuses to handle that Z8 IR chip.  This
allows us to assume that ir->tx != NULL in a few places in the driver,
and also allows us to always report Tx is available to userspace.
Get rid of unneeded tests for ir->tx == NULL and always report that
Tx is available.

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