net: linkmode: make linkmode_test_bit() take const pointer

linkmode_test_bit() does not modify the address; test_bit() is also
declared const volatile for the same reason. There's no need for
linkmode_test_bit() to be any different, and allows implementation of
helpers that take a const linkmode pointer.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/linkmode.h

@@ -71,7 +71,7 @@ static inline void linkmode_change_bit(int nr, volatile unsigned long *addr)
 	__change_bit(nr, addr);
 }
 
-static inline int linkmode_test_bit(int nr, volatile unsigned long *addr)
+static inline int linkmode_test_bit(int nr, const volatile unsigned long *addr)
 {
 	return test_bit(nr, addr);
 }