n_r3964: fix lock imbalance

There is omitted BKunL in r3964_read.

Centralize the paths to one point with one unlock.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>