V4L/DVB: dvb: fix smscore_getbuffer() logic

Drivers shouldn't sleep while holding a spinlock. A previous workaround
were to release the spinlock before callinc schedule().

This patch uses a different approach: it just waits for the
siano hardware to answer.

Signed-off-by: Richard Zidlicky <rz@linux-m68k.org>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>