rt2x00: Restrict DMA to 32-bit addresses.

None of the rt2x00 PCI devices support 64-bit DMA addresses (they all
only accept 32-bit buffer addresses). Hence it makes no sense to try to
enable 64-bit DMA addresses. Only try to enable 32-bit DMA addresses.

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