hwmon: (w83795) Delay reading pwm config registers

Wait until we need the pwm config register values, instead of
pre-reading them. This saves over 1 second on modprobe on my test
system.

Obviously this time is added when first accessing pwm config
attributes, however not everybody will use them, so it seems unfair
to slow down driver loading (and thus boot) for an optional feature.

Signed-off-by: Jean Delvare <khali@linux-fr.org>