USB: serial: fix potential NULL-dereference at probe

Make sure we have at least one port before attempting to register a
console.

Currently, at least one driver binds to a "dummy" interface and requests
zero ports for it. Should such an interface also lack endpoints, we get
a NULL-deref during probe.

Fixes: e5b1e206 ("USB: serial: make minor allocation dynamic")
Cc: stable <stable@vger.kernel.org>	# 3.11
Signed-off-by: Johan Hovold <johan@kernel.org>