Fix NE2000 driver:

Fixed typo in ne2000.h, thinko re n2k_inb() usage, don't try
to do anything in eth_stop() if eth_init() was not called.
Simplified RX path in order to avoid timeouts on really really
fast NE2000 cards (read: qemu with internal tftp), NetLoop() is
clever enough to cope with 1 packet per eth_rx().

Signed-off-by: Vlad Lungu <vlad@comsys.ro>