net: introduce per device index

Instead of counting the device index everytime a functions needs it, store
it in the eth_device struct. eth_register() keeps track of the indices and
updates the device's index number. This simplifies some functions in
net/eth.c.

Additionally, a network driver can now query its index, eg. to get the
correct environment ethaddr name.

Signed-off-by: Michael Walle <michael@walle.cc>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Mike Frysinger <vapier@gentoo.com>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>