mac80211: Remove unused third address from mesh address extension header.

The Mesh Control header only includes 0, 1 or 2 addresses. If there is
one address, it should be interpreted as Address 4.  If there are 2,
they are interpreted as Addresses 5 and 6 (Address 4 being the 4th
address in the 802.11 header).

The address extension used to hold up to 3 addresses instead of the current 2.
I'm not sure which draft version changed this, but it is very unlikely that it
will change again given the state of the approval process of this draft.  See
section 7.1.3.6.3 in current draft (8.0).

Also, note that the extra address that I'm removing was not being used, so this
change has no effect on over-the-air frame formats.  But I thought I better
remove it before someone does start using it.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>