drivers/net/at91_emac.c: change return values

Some return values pretended correct pass. This patch changes them according
to README.drivers.net. This patch changes e.g. command 'dhcp' to stop after
errorneous autonegotiation.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>