V4L/DVB (4849): Add missing spin_unlock to saa6588 decoder driver

Sparse noticed a lock imbalance in read_from_buf(). Further inspection shows
that the lock should not be held when the function exits.
This adds a spin_unlock_irqrestore(), so that every exit path of the
read_from_buf() function is consistent. The unlock was missing on an error
path.

Signed-off-by: Ira W. Snyder <devel@irasnyder.com>
Signed-off-by: Hans J. Koch <koch@hjk-az.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>