staging: brcm80211: removed functionality to disable N mode

NVRAM contains a parameter that can disable N mode. This functionality
is not needed. As a consequence, brcms_c_protection_upd() could be
simplified.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>