[TG3]: Replace some magic 5704S constants

This patch replaces magic values with preprocessor definitions for
the sg_dig_ctrl and sg_dig_status registers.  This is preparatory work
for the next patch.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>