[PATCH] zd1211rw, fix oops when ejecting install media

The disconnect function can dereference the net_device structure when it
is never allocated. This is the case when ejecting the device installer.

Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
Acked-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>