net/fec: improve pm for better suspend/resume

The following commit made a fix to use fec_enet_open/fec_enet_close
over fec_enet_init/fec_stop for suspend/resume, because fec_enet_init
does not allow to have a working network interface at resume.

  e3fe8558
  net/fec: fix pm to survive to suspend/resume

This fix works for i.mx/mxc fec controller, but fails on mx28 fec
which gets a different interrupt logic design. On i.mx fec, interrupt
can be triggered even bit ETHER_EN of ECR register is not set. But
on mx28 fec, ETHER_EN must be set to get interrupt work. Meanwhile,
MII interrupt is mandatory to resume the driver, because MDIO
read/write changed to interrupt mode by commit below.

  97b72e43
  fec: use interrupt for MDIO completion indication

fec_restart/fec_stop comes out as the solution working for both
cases.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>