Staging: rt2860sta: prevent a panic when disabling when associated

This fixes a panic which is triggered when the hardware "disappears" from
beneath the driver, i.e. when wireless is toggled off via Fn-F2 on various
EeePC models.

Ref. bug report http://bugzilla.kernel.org/show_bug.cgi?id=13390
          panic http://bugzilla.kernel.org/attachment.cgi?id=21928

Signed-off-by: Darren Salt <linux@youmustbejoking.demon.co.uk>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>