USB: serial: io_edgeport: fix up switch fall-through comments

Gustavo has been working to fix up all of the switch statements that
"fall through" such that we can eventually turn on
-Wimplicit-fallthrough.  As part of that, the io_edgeport.c driver is a
bit "messy" with the parsing logic of a data packet.  Clean that logic
up a bit by unindenting one level of the logic, and properly label
/* Fall through */ to make gcc happy.

Reported-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Johan Hovold <johan@kernel.org>