rtl8xxxu: Split filling of TX descriptors into separate functions

Split the filling of TX descriptors into a generic portion used on all
devices, and format specific helper functions provided in the fops
structure.

This also cleaned up some mess, even if non harmful, in the handling
of txdesc40 descriptors, where the code randomly would switch between
the pointer to tx_desc and tx_desc40.

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