staging: brcm80211: return error code to mac80211 for 40MHz channels

When mac80211 attempts to configure the driver for 40MHz channel it
will return an error code -EIO as this is not yet supported.

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