ethernet: defer NIC receive injection to hsync

ethernet: defer NIC receive injection to hsync

A2065/Ariadne injected packets directly from the host network
callback. With SLIRP or pcap this callback can run on a helper
thread, so receive DMA and interrupt updates could race PPC I/O
callbacks.

Queue received frames and drain them from the device hsync
handler, where the rest of the emulated card state is updated. Also
stop the NE2000 host callback from draining its queue, leaving that
work to the existing hsync path.