rtl8xxxu: rtl8xxxu_h2c_cmd(): Add size argument

The firmware command API differs slightly between new and old
devices. The new generation requires the size since there is no
extension bit encoded into the command number.

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