Correctly limit pending requests

[CTurt]

Failing for me due to accumulating large number of pending requests (over 30,000) and then printing libusb error.

Root cause is logic error in HandleEvents:

https://github.com/hcs64/gs_libusb/blob/45e7b5f254c2f7816d651cc7a18a6c37ba1e15b2/gscomms/gscomms.c#L708

void HandleEvents(gscomms * g, long timeout_ms, int max_pending_writes) {
    if (g->writes_pending > max_pending_writes) {

Changing this if to be a while makes pending requests stablize at 256, slows down progress bar, but actually succeeds eventually.