b43: Dereference of wl->current_dev must be protected by wl->mutex

Put all access to wl->current_dev under protection of the mutex.

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