ftgmac100: Increase reset timeout

We had reports of 50us not being sufficient to reset the MAC,
thus hitting the "Hardware reset failed" error bringing the
interface up on some AST2400 based machines.

This bumps the timeout to 200us.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>