rt2x00: rt2800lib: correctly set HT20/HT40 filter

Simple patch to correct HT20/HT40 filter setting.
Tested with Rt3290, Rt3352 and Rt5350

Signed-off-by: Serge Vasilugin <vasilugin@yandex.ru>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>