staging: brcm80211: remove unused function from wlc_stf.c

Working through a list of unused functions in the driver tree.
This file has following redundant function(s):

wlc_stf_stbc_rx_get
wlc_stf_rxchain_set

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