tg3: Pull phy int lpbk setup into separate func

This patch pulls out the internal phy loopback setup code into a
separate function.  This cleans up the loopback test code and makes it
available for NETIF_F_LOOPBACK support later.

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