b43: Remove TX spinlock

This removes the TX spinlock and defers TX to a workqueue to allow
locking wl->mutex instead and to allow sleeping for register accesses.

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>