usbip: Fix implicit fallthrough warning

GCC 7 now warns when switch statements fall through implicitly, and with
-Werror enabled in configure.ac, that makes these tools unbuildable.

We fix this by notifying the compiler that this particular case statement
is meant to fall through.

Reviewed-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Jonathan Dieter <jdieter@lesbg.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>