bcm570x: 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.

Since the address is in the PLM_DEVICE_BLOCK structure already, there is
no need to pass the NodeAddress as a second parameter.  So drop the second
argument to the LM_SetMacAddress() function (and update the tigon3 driver
accordingly).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Ben Warren <biggerbadderben@gmail.com>