rtl8xxxu: Correct rxdesc16 definition

This corrects the definition of rxdesc16 to correctly specify pkt_cnt
for aggregated packets. This is based on the code of the vendor
rtl8723au driver, as opposed to the struct definitions they use.

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