staging: brcm80211: remove dead code from brcmfmac

brcmf_sdcrad_cfg_read_word and brcmf_sdcard_cfg_write_word are
not used by anyone.

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>