usb: class specific interface requests

Mass Storage Reset and Get Max LUN are class specific requests, but
they were not marked as such in hw/usb-msd.c, moved therefore
ClassInterfaceRequest and ClassInterfaceOutRequest from hw/usb-net.c
to hw/usb.h.
Furthermore there was a problem in hw/usb-ohci.c when using DEBUG
concerning systems where size_t is a 32 bit integer (printf resulted
in a segmentation fault).

Signed-off-by: Max Reitz <max@tyndur.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>