staging: brcm80211: remove unused functions from sbutils.c

Cleaning up unused function from the driver sources. This
file contained the following unused functioin(s):

sb_base
sb_taclear
sb_serr_clear

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>