wireless: b43: fix Oops on card eject during transfer

An Oops has once been observed, when the SDIO card had been ejected during
IO. The PC value shows, that the dev pointer in b43_op_stop() was NULL.

(I moved the NULL check before the lock, based upon a suggestion from
Julian Calaby <julian.calaby@gmail.com>. -- JWL)

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>