rt2x00: usb: fix reset resume

Patch fixes warnings like below happened on resume:

WARNING: at net/mac80211/driver-ops.h:12 check_sdata_in_driver+0x32/0x34()

Problem is that in __ieee80211_susped() we remove sdata (i.e wlan0
interface) and then during resume we call usb_unbind_interface() ->
ieee80211_unregister_hw() with sdata removed.

Patch fixes problem by adding .reset_resume calback, hence we do not
unbind usb device on resume. This callback can be the same as normal
.resume callback, sice we do all needed initalization during interface
start, which is performed on resume [ ieee80211_resume() ->
ieee80211_reconfig() -> rt2x00mac_start() -> rt2x00lib_start ].

Resolves:
https://bugzilla.kernel.org/show_bug.cgi?id=48041



Reported-by: David Herrmann <dh.herrmann@googlemail.com>
Reported-and-tested-by: Stephen Boyd <bebarino@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>