ar9170: fix struct layout on arm

arm will pad even between u8's, so mark the structs/unions
packed. Fixes a build bug on arm due to BUILD_BUG_ON tests
in the code.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>