b43: Remove PIO RX workqueue

This removes the PIO RX work. It's not needed anymore, because
we can sleep in the threaded interrupt handler.

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>