[IRDA]: Initial support for MCS7780 based dongles

The MosChip MCS7780 chipset is an IrDA USB bridge that
doesn't conform with the IrDA-USB standard and thus needs
its separate driver.
Tested on an actual MCS7780 based dongle.

Original implementation by Brian Pugh <bpugh@cs.pdx.edu>

Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>