mac80211: make alignment warning optional

Driver authors should be aware of the alignment requirements, but
not everybody cares about the warning. This patch makes it depend
on a new Kconfig symbol MAC80211_DEBUG_PACKET_ALIGNMENT which can
be enabled regardless of MAC80211_DEBUG and is recommended for
driver authors (only). This also restricts the warning to data
packets so other packets need not be realigned to not trigger the
warning.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>