usb_rndis: fix handling of multiple packets in one USB transaction

I got my pointer math wrong because some things in RNDIS use uint32 as
the base, but some things are in bytes. Most of the time this would result
in an offset past the end of the USB buffer, so it would just lead to
ignoring all but the first packet. But if the first packet was small enough,
it would point somewhere still in the buffer, and we would read the wrong
data.

Fixes #17775

Change-Id: I32ec0081336b1f772d4dc3099a0ac2c691aa12f0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5377
Reviewed-by: Adrien Destugues <[email protected]>

src/add-ons/kernel/drivers/network/ether/usb_rndis/RNDISDevice.cpp

@@ -316,7 +316,9 @@ RNDISDevice::Read(uint8 *buffer, size_t *numBytes)
 		fReadHeader[1], fReadHeader[2], fReadHeader[3]);
 
 	// Advance to next packet
-	fReadHeader += fReadHeader[1];
+	fReadHeader = (uint32*)((uint8*)fReadHeader + fReadHeader[1]);
+
+	// Are we past the end of the buffer? If so, prepare to receive another one on the next read
 	if ((uint32)((uint8*)fReadHeader - fReadBuffer) >= fActualLengthRead)
 		fReadHeader = NULL;