[PATCH] dvb: dst: protect dst_write_tuna from simultaneous writes

dst_write_tuna needs to be protected against simultaeneous writes, just like
dst_command

Signed-off-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>