staging: usbip: fix potential segfault because of unchecked return value of strchr.

This doesn't happen with the usbip virtual hci module, but another
module wanting to interface with this user space code could cause a
seg-fault by sending data without newlines.

Signed-off-by: Christopher Harvey <charvey@matrox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>