rt2x00: fix rfkill regression on rt2500pci

As reported by Niels, starting rfkill polling during device probe
(commit e2bc7c5f, generally sane change) broke rfkill on rt2500pci
device. I considered that bug as some initalization issue, which
should be fixed on rt2500pci specific code. But after several
attempts (see bug report for details) we fail to find working solution.
Hence I decided to revert to old behaviour on rt2500pci to fix
regression.

Additionally patch also unregister rfkill on device remove instead
of ifconfig down, what was another issue introduced by bad commit.

Bug report:
https://bugzilla.kernel.org/show_bug.cgi?id=73821



Fixes: e2bc7c5f ("rt2x00: Fix rfkill_polling register function.")
Cc: stable@vger.kernel.org
Bisected-by: Niels <nille0386@googlemail.com>
Reported-and-tested-by: Niels <nille0386@googlemail.com>
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>