Revert "media: dvbsky: use just one mutex for serializing device R/W ops"

Revert "media: dvbsky: use just one mutex for serializing device R/W ops"

As pointed at:
	https://bugzilla.kernel.org/show_bug.cgi?id=199323

This patch causes a bad effect on RPi. I suspect that the root
cause is at the USB out of tree RPi driver, with uses high priority
interrupts instead of normal ones. Anyway, as this patch
is mostly a cleanup, better to revert it.

This reverts commit 7d95fb746c4eece67308f1642a666ea1ebdbd2cc.

Cc: stable@vger.kernel.org # For Kernel 4.18
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>