wireless: Kconfig: add missing dependency for airo_cs

commit 4c59ff22 "wireless: Kconfig: add missing dependency" added a number
of 'depends on CFG80211' statements, but missed the AIRO_CS driver that
also causes the airo.c file to be built. This adds the (hopefully) last
such missing statement

Cc: "Zhao, Gang" <gamerh2o@gmail.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>