usb: usb_transfer.c: Address `-Wextra` warnings

Changed incrementing `for` loop counters to `size_t` from `int` to
eliminate warning, "warning: comparison of integer expressions of
different signedness: 'uint32_t' {aka 'unsigned int'} and 'int'
[-Wsign-compare]"

Signed-off-by: Zachary J. Fields <zachary_fields@yahoo.com>