rtl8xxxu: Fix big-endian problem reporting mactime

The full RX descriptor is converted so converting tsfl again would
return it to it's original endian value.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Cc: stable@vger.kernel.org # 4.8+
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>