zd1211rw: fix unaligned access in zd_mac_rx

Fix an unaligned memory access in the zd_mac_rx function of zd1211rw
that causes problems on SPARC64.

Signed-off-by: Patrick Simmons <linuxrocks123@netscape.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>