rt2800: disable PS by default on USB

We have disabled it currently on other buses. PS can cause some issues,
not necessarily with our driver but on AP, that are not easy to debug.
Since behaviour differs on rt2800usb and rt2800pci, user usually blame
for malfunction rt2800usb driver, whereas issue is on AP side.

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>