rt2x00: Disable powersaving by default in rt2500usb.

Recent bug reports have shown that rt2500usb also suffers from the
powersave problems that the PCI rt2x00 drivers suffer from.
So disable powersaving by default for rt2500usb as well.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>