net: Set LL_MAX_HEADER properly for wireless.

Wireless networking, particularly with MESH enabled, has
quite strong requirements for link-layer header space.

Based upon some numbers and descriptions from Johannes Berg
we use 96 (same as AX25) for plain wireless, and with
mesh enabled we use 128.

In the process, simplify the cpp conditional logic here by
ordering the cases by those needing the most space down
to those needing the least case.

Signed-off-by: David S. Miller <davem@davemloft.net>