USB: serial: io_edgeport: fix epic-descriptor handling

Use a dedicated buffer for the DMA transfer and make sure to detect
short transfers to avoid parsing a corrupt descriptor.

Fixes: 6e8cf775

 ("USB: add EPIC support to the io_edgeport driver")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>