tg3: Create critical section around GPIO toggling

The code that performs the power source switching will need to consider
the status of the other devices before making any switches.  The status
updates and power source switching will need to be an atomic operation,
so a critical section will be needed.  This patch establishes the
critical section through a CPMU mutex.

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