Currently the smsc911x driver has mininal power management: during
driver probe, the device is powered up, and during driver remove, it is
powered down.

Improve power management by making it more fine-grained:
  1. Power the device down when driver probe is finished,
  2. Power the device (down) when it is opened (closed),
  3. Make sure the device is powered during PHY access.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210118150857.796943-1-geert+renesas@glider.beSigned-off-by: Jakub Kicinski <kuba@kernel.org>