USB: serial: add support for serial port on the moschip 7715

Add support for the serial port on devices based on the MosChip 7715,
which provides a serial and parallel port on a single usb interface.
This is added to the existing driver for the Moschip 7720 dual serial
port device.  The 7715 is very similiar to the 7720, requiring only the
addition of a calc_num_ports() function, a separate interrupt-in
endpoint callback, and some manipulation of the port pointers added to
the attach() function to correct the fact that the usbserial core
erroneously assigns the first bulk in/out endpoint pair to the serial
port (the 7715 uses these for its parallel port).  There is no support
for the 7715's parallel port yet.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>