Support up to 4 virtual APs and as many virtual STA interfaces
as desired.

This patch is ported forward from a patch that Patrick McHardy
did for me against 2.6.31.
Signed-off-by: NBen Greear <greearb@candelatech.com>
Acked-by: NBruno Randolf <br1@einfach.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>