at76c50x-usb: additional disconnect fixes

Additional attempts to fix Oops on disconnect, that appear to be successful.
However, some may be extraneous.

The cancel_delayed_work call is probably the most necessary.  The
device_unplugged check may not be necessary.  del_timer_sync may not
be necessary either, but the Oops I was receiving was related to
timers.  Hence the addition.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>