r8169: fix random mdio_write failures

Some configurations need delay between the "write completed" indication
and new write to work reliably.

Realtek driver seems to use longer delay when polling the "write complete"
bit, so it waits long enough between writes with high probability (but
could probably break too). This patch adds a new udelay to make sure we
wait unconditionally some time after the write complete indication.

This caused a regression with XID 18000000 boards when the board specific
phy configuration writing many mdio registers was added in commit
2e955856 (r8169: phy init for the 8169scd). Some of the configration
mdio writes would almost always fail, and depending on failure might leave
the PHY in non-working state.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Acked-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>