rtl8xxxu: Add definition for 8723bu tx descriptor

Newer generation chips use a 40 byte TX descriptor, compared to the
32 byte descriptor used on older chips.

This adds the definition for the 40 byte descriptor.

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