[IRDA]: mcs7780 needs to free allocated rx buffer.

While testing the mcs7780 based IrDA USB dongle I've stumbled upon
memory leak in mcs_net_close(). Patch below fixes it.

Signed-off-by: Hinko Kocevar <hinko.kocevar@cetrtapot.si>
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>