staging: brcm80211: implement mac80211 callback set_rts_threshold

adds implementation for allowing configuration of dot11RTSThreshold
as defined in the 802.11 standards. The mac80211 module will use
callback set_rts_threshold to configure this in the driver.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Brett Rudley <brudley@broadcom.com>
Reviewed-by: Henry Ptasinski <henryp@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>