zd1211rw: Use DMA-aware buffer for usb transfer

The zd1211rw driver uses unaligned stack buffer for USB control
message.  But it might cause stack corruption on non-coherent
platform, such as MIPS.  Use DMA-aware buffers for USB transfer.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: John W. Linville <linville@tuxdriver.com>