staging: rtl8723au: Get rid of SetSeqNum()

In every last use case, the header was just cleared with memset() so
there is no need to use the complex SetSeqNum() macro when
cpu_to_le16(IEEE80211_SN_TO_SEQ()) output will do just fine.

In addition, get rid of __le16 *fctrl usage when there is no need for
it.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>