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

* in the actual driver, calling fec_stop and fec_enet_init doesn't
allow to have a working network interface at resume (where a
ifconfig down and up is required to recover the interface)
* by using fec_enet_close and fec_enet_open, this patch solves this
problem and handle the case where the link changed between suspend
and resume
* this patch also disable clock at suspend and reenable it at resume

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: David S. Miller <davem@davemloft.net>