USB: serial: qcserial: fix altsetting probing

Drivers should not assume that interface descriptors have been parsed in
any particular order so use the interface number to look up the second
alternate setting. That number is also what the driver later use to
switch setting.

Note that although the driver could end up verifying the existence of
the expected endpoints on the wrong interface, a later sanity check in
usb_wwan_port_probe() would have caught this before it could cause any
real damage.

Fixes: a78b4282

 ("USB: serial: add qualcomm wireless modem driver")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>