rtl8xxxu: Change name of struct tx_desc to be more decriptive

There are two major types of TX descriptor formats for the RTL parts,
the old 32 byte descriptor, and the newer 40 byte descriptor used by
the 8723bu, 8192eu, and 88xx series.

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