[PATCH] softmac: unified capabilities computation

This patch moves the capabilities field computation to a function for clarity
and adds some previously unimplemented bits.

Signed off by Joseph Jezak <josejx@gentoo.org>
Signed-off-by: Daniel Drake <dsd@gentoo.org>
Acked-By: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>