[media] lirc: use kfifo_initialized() on lirc_buffer's fifo

[media] lirc: use kfifo_initialized() on lirc_buffer's fifo

We can use kfifo_initialized() to check if the fifo in lirc_buffer is
initialized or not. There's no need to have a dedicated fifo status
variable in lirc_buffer.

[m.chehab@samsung.com: add the same change to lirc_zilog, to avoid
 breaking compilation of staging drivers]
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>