dev/usb: unsigned some loop indexes.

Although usually small, values produced by nitems() are unsigned.
By unsigning the corresponding indexes we avoid signed vs unsigned
comparisons. This may have some effect on performance, although given the
small sizes the effect will not be perceivable, and it makes the code
clearer.

Respect the style of the changed files: one uses u_int while the other
uses "unsigned int".

Reviewed by: hselasky