b43: Fix possible unaligned u32 access

Fix possible unaligned u32 access in b43_generate_plcp_hdr().
Unaligned data is read/write with a u32 pointer instead of using the
packed structure. Some versions of gcc ignore the "packed" attribute, if the
structure element is accessed through a local pointer.

Signed-off-by: Matthieu CASTET <castet.matthieu@free.fr>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>