We can only use the madc to check for 'ac' availability if the madc has
been compiled in.  If not: assume always using USB.
Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: NeilBrown <neil@brown.name>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>