rtl8xxxu: Enable monitor mode by handling filters

Monitor mode is enabled by handling the filter flags we get from mac80211 in
rtl8xxxu_configure_filter() and writing them to the RCR register.

By handling the filters, we can also stop setting the BSSID filters in the
association event.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>