staging: brcm80211: remove 'used' keyword from function ai_ispcie()

The function ai_ispcie() is a static function that is currently being
called from the c sources in the driver. Side-effect of the keyword
is that warning will suppressed when this function is no longer used.
As there is no assembly code using this function, the keyword is removed.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>