staging: usbip: bugfix add number of packets for isochronous frames

The number_of_packets was not transmitted for RET_SUBMIT packets. The
linux client used the stored number_of_packet from the submitted
request. The windows userland client does not do this however and needs
to know the number_of_packets to determine the size of the transmission.

Signed-off-by: Arjan Mels <arjan.mels@gmx.net>
Cc: Takahiro Hirofuchi <hirofuchi@users.sourceforge.net>
Cc: Max Vozeler <max@vozeler.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>