tg3: Create funcs for power source switching

The power source switching code is about to get a little more complex.
This patch seeks to simplify future power source switching patches by
clarifying the existing code.

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@conan.davemloft.net>