b43: remove SHM spinlock

This removes the SHM spinlock.
SHM is protected by wl->mutex.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>