tg3: Add macro for SMDSP toggling

A common AUX CTRL operation in the driver is to enable and disable the
SMDSP.  This patch consolidates the code so that the details of the
operation are in one place.  This patch also adds code to make sure the
SMDSP is enabled before executing code that relies on it.

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