lan91c96/smc91111/smc911x: get mac address from environment

The environment is the canonical storage location of the mac address, so
we're killing off the global data location and moving everything to
querying the env directly.

Also, do not bother checking the EEPROM if the env is setup.  This
simplifies the code greatly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Wolfgang Dnek <wd@denx.de>
CC: Ben Warren <biggerbadderben@gmail.com>
CC: Rolf Offermanns <rof@sysgo.de>
CC: Erik Stahlman <erik@vt.edu>
CC: Daris A Nevil <dnevil@snmc.com>
CC: Sascha Hauer <s.hauer@pengutronix.de>