staging: brcm80211: separated public from private ioctl functions

net_device ioctl handler was called both by the OS as by the driver
itself. Split the ioctl handler into two functions to make code paths
more clear.

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