staging: brcm80211: move rssi computation to place we need it

The rssi computation was done upon getting the frame from the
queue, but the value is needed when filling in the receive status
data for mac80211. The call to wlc_phy_rssi_compute() has been
deferred.

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>