The parameter rhport is unsigned so there is no need checking for
negative values.

This will remove the following warning message when compiling with "make W=1 ...":
  drivers/staging/usbip/vhci_sysfs.c: In function ‘valid_args’:
  drivers/staging/usbip/vhci_sysfs.c:138: warning: comparison of unsigned expression < 0 is always false
Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>