staging: brcm80211: removed fullmac function brcmf_sdcard_iovar_op()

brcmf_sdcard_iovar_op was only called with iovar "sd_rxchain", and
always returns 'false' as the queried value. Thus, the entire
function, related iovar tables and related functions could be
removed.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>