usbnet: add a mutex around phy register access

When working on the mcs7830, I noticed the need for a mutex in its
mdio_read/mdio_write functions. A related problem seems to be present
in the asix driver in the respective functions.

This introduces a mutex in the common usbnet driver and uses it
from the two hardware specific drivers.

Acked-by: David Hollis <dhollis@davehollis.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/net/usbnet.c

@@ -1144,6 +1144,7 @@ usbnet_probe (struct usb_interface *udev, const struct usb_device_id *prod)
 	dev->delay.function = usbnet_bh;
 	dev->delay.data = (unsigned long) dev;
 	init_timer (&dev->delay);
+	mutex_init (&dev->phy_mutex);
 
 	SET_MODULE_OWNER (net);
 	dev->net = net;