tg3: Add ethtool_eee struct and tg3_setup_eee()

Add an eee structure and update it with eee settings. This will be used
for set/get_eee operations. Add common function tg3_setup_eee() that
will be used in the subsequent patches.

Reviewed-by: Ben Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>