macb: Remove inappropriate spinlocks around mii calls

Remove spin_lock_irqsave() around mii_ethtool_gset, mii_ethtool_sset
and generic_mii_ioctl. These are unnecessary and harmful because
the mii calls may call back into the mdio functions, which may sleep.

Pointed out by David Brownell.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>