rtl8xxxu: Add more 40 byte TX desc bit definitions

Add additional bit definitions for 40 byte TX descriptors, and rename
bits for 32 byte descriptors that are located differently in the 40
byte descriptor format.

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