api: net: fix length check in eth_receive()

If the requested length is too small to hold the received packet,
eth_receive() will return -1 and will leave the packet in the receive
buffers. Instead of returning an error in this case, we return the first
portion of the received packet and remove it from the receive buffers.

This fixes FreeBSD's ubldr. Without this patch it will just stop receiving
packets if the NIC receives more than PKTBUFSRX too large packets.

Signed-off-by: Michael Walle <michael@walle.cc>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Cc: Rafal Jaworowski <raj@semihalf.com>
Cc: Piotr Kruszynski <ppk@semihalf.com>