rtl8xxxu: Split USB quirks into gen1 and gen2 quirks

This removes a bunch of if () spaghetti and re-applies the USB bus
quirks for 8188/8192 that had gotten lost.

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