staging: brcm80211: remove unused function from wlc_channel.c

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

wlc_channel_country_abbrev
wlc_channel_locale_flags
wlc_channel_get_chanvec
wlc_valid_40chanspec_in_band
wlc_channel_set_txpower_limit
wlc_valid_chanspec

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>