This patch adds IEEE80211_MAX_FRAME_LEN which is useful for drivers trying
to determine how much to allocate for their RX buffers.

It also updates the comment on IEEE80211_MAX_DATA_LEN based on revisions
in 802.11e.

IEEE80211_MAX_FRAG_THRESHOLD and IEEE80211_MAX_RTS_THRESHOLD are also
revised due to the new maximum frame size.
Signed-off-by: NMichael Wu <flamingice@sourmilk.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>