[PATCH] dvb: dst: protect the read/write commands with a mutex

We need to protect the read/write commands with a mutex.

Bug reported by Henrik Sjoberg <henke@epact.se>

Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Cc: Johannes Stezenbach <js@linuxtv.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>