serial: bfin_5xx: add missing spin_lock init

The Blackfin serial driver never initialized the spin_lock that is part of
the serial core structure, but we never noticed because spin_lock's are
rarely enabled on UP systems.  Yeah lockdep and friends.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>