USB: qcserial: fix port handling on Gobi 1K and 2K+

Bjorn's latest patchset does break Gobi 1K and 2K because on both
devices as it claims usb interface 0.  That's because usbif 0 is not
handled in the switch statement, and thus the if0 gets claimed when it
should not.  So let's just make things even simpler yet, and handle both
the 1K and 2K+ cases separately.  This patch should not affect the new
Sierra device support, because those devices are matched via
interface-specific matching and thus should never hit the composite
code.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>