staging: usbip: Fix format string mismatch in usbip_vhci_attach_device2

Argument type of sockfd is set as int, but format string
is set as unsigned int. Fix the mismatch.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>