[media] rc-core: add trailing silence in rc-loopback tx

If an IR command is sent (using the LIRC userspace) to rc-loopback
which doesn't include a trailing space, the result is that the message
won't be completely decoded. In addition, "leftovers" from a previous
transmission can be left until the next one. Fix this by faking a long
silence after the end of TX data.

Signed-off-by: David Härdeman <david@hardeman.nu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>