b43: fix invalid memory access in b43_ssb_remove()

wldev is freed in b43_one_core_detach() and should not be accessed after
that call.  Keep wldev->dev in a local variable.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>