Delay sending LINK_STATE_UP to prevent dropping packets on I35[04] and I21[01].

Some (not all) systems use I35[04] or I21[01] don't send packet soon
after linkup. The MAC send a packet to the PHY and any error is not
observed. This behavior causes a problem that gratuitous ARP and/or
IPv6 DAD packet are silently dropped. To avoid this problem, don't
call mii_pollstat() here which will send LINK_STATE_UP notification
to the upper layer. Instead, mii_pollstat() will be called in
wm_gmii_mediastatus() or mii_tick() will be called in wm_tick().

Note that the similar workaround is in Linux's igb driver though it's
only for I21[01].

OK'd by hikaru@ and knakahara@.

(msaitoh)