e1000: Make PHY powerup/down a function

In relation to the irq work done earlier we also move the PHY powerup
and powerdown functions into separate functions and move the calls to
_close and _open, making the PHY stay in it's power state as long as
the device is _up.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>