staging: brcm80211: remove usage of macro WL11N_20MHZONLY

The macro WL11N_20MHZONLY is never defined so code #ifdef'ed by
it has been removed.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>