usbip: remove check for negative values for an unsigned value

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>