USB: serial: mos7720: fix parallel probe

A static usb-serial-driver structure that is used to initialise the
interrupt URB was modified during probe depending on the currently
probed device type, something which could break a parallel probe of a
device of a different type.

Fix this up by overriding the default completion callback for MCS7715
devices in attach() instead. We may want to use two usb-serial driver
instances for the two types later.

Fixes: fb088e33

 ("USB: serial: add support for serial port on the
moschip 7715")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>