USB: add USB serial mos7720 driver

Add support for Moschip 7720 USB dual port usb to serial device.

This driver is originally based on the drivers/usb/io_edgeport.c driver.

Cleaned up and forward ported by me.

Cc: VijayaKumar <vijaykumar@aspirecom.net>
Cc: AjayKumar <ajay@aspirecom.net>
Cc: Gurudeva <gurudev@aspirecom.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>