ath5k: Allow ath5k to support virtual STA and AP interfaces.

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: Ben Greear <greearb@candelatech.com>
Acked-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>