staging: usbip: Renaming pr_warning function

Eliminated the following warning "WARNING: Prefer pr_warn(... to
pr_warning(...)" by renaming the pr_warning function to pr_warn in
usbip_event.c file.

Signed-off-by: Lisa Nguyen <lisa@xenapiadmin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>