tg3: Fix MII_TG3_DSP_EXP8 offset

The offset to the MII_TG3_DSP_EXP8 register is incorrect.  Fix it.

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