3c59x: Use fine-grained locks for MII and windowed register access

This avoids scheduling in atomic context and also means that IRQs
will only be deferred for relatively short periods of time.

Previously discussed in:
http://article.gmane.org/gmane.linux.network/155024



Reported-by: Arne Nordmark <nordmark@mech.kth.se>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>