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>