tg3: Restructure tg3_test_loopback

The tg3_test_loopback() function is starting to get more complicated as
more loopback tests are added.  This patch cleans up the code.

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