b43: Don't abuse wl->current_dev in the led work

Don't abuse wl->current_dev in the LED work for checking whether we're
going down. Add an explicit variable.
This fixes a crash on rmmod dereferencing the wl->current_dev NULL pointer
in various other places of the driver.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>