remove "num_iso_packets" from structure "struct usbi_transfer"

[lerdmann]

"num_iso_packets" is defined in both, "struct usbi_transfer" and also in "struct libusb_transfer".

This can lead to a clash as function "libusb_alloc_transfer" will set the value in "struct usbi_transfer" whereas function "libusb_fill_iso_transfer" will set the value in "struct libusb_transfer" which can lead to a mismatch.

I suggest to change "libusb_alloc_transfer" to :

DEFAULT_VISIBILITY struct libusb_transfer * LIBUSB_CALL libusb_alloc_transfer(int iso_packets)
{
size_t priv_size;
size_t alloc_size;
unsigned char *ptr;
struct usbi_transfer *itransfer;
struct libusb_transfer *transfer;

assert(iso_packets >= 0);
if (iso_packets < 0)
	return NULL;

priv_size = PTR_ALIGN(usbi_backend.transfer_priv_size);
alloc_size = priv_size
	+ sizeof(struct usbi_transfer)
	+ sizeof(struct libusb_transfer)
	+ (sizeof(struct libusb_iso_packet_descriptor) * (size_t)iso_packets);
ptr = calloc(1, alloc_size);
if (!ptr)
	return NULL;

itransfer = (struct usbi_transfer *)(ptr + priv_size);
itransfer->priv = ptr;
usbi_mutex_init(&itransfer->lock);
transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
usbi_dbg("transfer %p", transfer);
    if (transfer) transfer->num_iso_packets = iso_packets;
return transfer;
}

and to remove parameter "num_iso_packets" from structure "struct usbi_transfer" and also remove parameter "num_iso_packets" from function "libusb_fill_iso_transfer" or ignore it (leave as deprecated). That will ensure that there is only one value for "num_iso_packets" and that this value will always be an integral part of the transfer (albeit zero for control,bulk and interrupt transfers) and not some internal control variable.

[mcuee]

@lerdmann Just wondering if you can create a pull request for this. Thanks.