staging: usbip: replaced pointer arithmetic, and strongly type function return.

Replaced pointer arithmetic by using array indexing, and changed
function return type for usbip_alloc_iso_desc_pdu from 'void*' to
'struct usbip_iso_packet_descriptor'.

Signed-off-by: Bart Westgeest <bart@elbrys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>