rtl8xxxu: Name RX descriptor types rxdesc16/rxdesc24

This caught a bug where too little memory was allocated for RX urbs
for parts using 24 byte RX descriptors

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