USB: serial: io_edgeport: fix epic-descriptor handling

USB: serial: io_edgeport: fix epic-descriptor handling

BugLink: http://bugs.launchpad.net/bugs/1691369
commit e4457d9798adb96272468e93da663de9bd0a4198 upstream.

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

Fixes: 6e8cf7751f9f ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>