V4L/DVB: IR: replace spinlock with mutex

Some handlers (lirc for example) allocates memory on initialization,
doing so in atomic context is cumbersome.
Fixes warning about sleeping function in atomic context.

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>