rtl8xxxu: Only setup USB interrupts for parts which support it

Only 1st generation chips do provide USB interrupts, so do not try to
setup interrupts for newer chips (8192eu and 8723bu).

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