staging: brcm80211: change interface for common rssi compute function

The function wlc_phy_rssi_compute() now uses d11rxhdr struct as
purely input parameter and returns the computed rssi value.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>