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>

diff --git a/drivers/usb/net/usbnet.h b/drivers/usb/net/usbnet.h
index 743947c..07c70ab 100644
--- a/drivers/usb/net/usbnet.h
+++ b/drivers/usb/net/usbnet.h
@@ -30,6 +30,7 @@
 	struct usb_device	*udev;
 	struct driver_info	*driver_info;
 	wait_queue_head_t	*wait;
+	struct mutex		phy_mutex;
 
 	/* i/o info: pipes etc */
 	unsigned		in, out;