[media] lirc_zilog: Remove disable_tx module parameter

The only reason to use the lirc_zilog module is for IR Tx, so remove
the possibility of disabling IR Tx.  If the user needs only IR Rx,
then the ir-kbd-i2c module works just fine, and doesn't require a
"firmware" image.

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