sh: reduce Migo-R smc91x overruns

Improve Migo-R ethernet performance by reducing smc91x overruns.
This is done by enabling SMC91X_NOWAIT and optimizing CS4 setup.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>