hwmon: (pmbus/adm1275) Add support for second current limit

ADM1275 supports a second current limit, which can be configured as either lower
or upper limit. Add support for it and report it as either lower or upper
critical current limit.

Also replace error return code EINVAL for unsupported pages with ENXIO as this
is more appropriate for the observed condition.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>