[media] media: lirc_dev: sanitize locking

Use the irctl mutex for all device operations and only use lirc_dev_lock
to protect the irctls array. Also, make sure that the device is alive
early in each fops function before doing anything else.

Since this patch touches nearly every line where the irctl mutex is
taken/released, it also renames the mutex at the same time (the name
irctl_lock will be misleading once struct irctl goes away in later
patches).

[mchehab@s-opensource.com: fix a merge conflict]
Signed-off-by: David Härdeman <david@hardeman.nu>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>