net/fec: fix the use of pdev->id

The pdev->id is used in several places for different purpose.  All
these uses assume it's always the id of fec device which is >= 0.
However this is only true for non-DT case.  When DT plays, pdev->id
is always -1, which will break these pdev->id users.

Instead of fixing all these users one by one, this patch introduces
a new member 'dev_id' to 'struct fec_enet_private' for holding the
correct fec device id, and replaces all the existing uses of pdev->id
with this dev_id.

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