mfd: update sm501 debugging/low information messages

Make the default output of the SM501 driver to be less noisy wrt to
message that have low informational value, or simply should be debug.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>