rtl8xxxu: Rename TX descriptor bits to map them to 32/40 byte descriptors

With the size based naming of TX descriptors. Change the bit
definition namings to indicate which descriptor format they match,
rather than having a device name in the bit name.

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