USB: serial: mos7720: clean up mcs7715 port setup

Clean up the mcs7715 port setup by using the new endpoint-remap
functionality provided by core. Instead of poking around in internal
port-structure fields, simply swap the endpoint descriptors of the two
ports in calc_num_ports before the port structures are even allocated.

Note that we still need to override the default interrupt completion
handler.

Signed-off-by: Johan Hovold <johan@kernel.org>