rtl8xxxu: byteswap the entire RX descriptor for 24 byte RX descriptors

This shouldn't affect little endian system, but may have prevented the
driver working on big endian systems for devices with the larger 24
byte RX descriptors.

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