media: dvbsky: use a single mutex and state buffers for all R/W ops

Re-use usb_mutex from dvb_usb_device for this.

Tested-by: Jan Pieter van Woerkom <jp@jpvw.nl>
Signed-off-by: Andrei Koshkosh <andreykosh000@mail.ru>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>