tg3: Extend loopback test timeout

This patch extends the loopback test timeout from 250 usec to 350 usec.
When the 5785 is paired against an AC131 phy, the older timeout is
just a little too close to the expected performance based on timings.

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