tc35815: Increase timeout for mdio

The current timeout value is too short for very high-load condition
which jiffies might jump up in busy-loop.
Also add minimum delay before checking completion of MDIO.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>