staging: rtl8723au: ioctl_cfg80211.c: Properly initialize seq_ctrl

Using SetSeqNum() on an uninitialized stack frame leaves us open to
leaving random fragment bits set in seq_ctrl in the frame header.

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