net: fec: don't reset irq coalesce settings to defaults on "ip link up"

Currently, when a FEC device is brought up, the irq coalesce settings
are reset to their default values (1000us, 200 frames). That's
unexpected, and breaks for example use of an appropriate .link file to
make systemd-udev apply the desired
settings (https://www.freedesktop.org/software/systemd/man/systemd.link.html

),
or any other method that would do a one-time setup during early boot.

Refactor the code so that fec_restart() instead uses
fec_enet_itr_coal_set(), which simply applies the settings that are
stored in the private data, and initialize that private data with the
default values.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>