hwmon: (pmbus/bpa-rs600) Don't use rated limits as warn limits

hwmon: (pmbus/bpa-rs600) Don't use rated limits as warn limits

In the initial implementation a number of PMBUS_x_WARN_LIMITs were
mapped to MFR fields. This was incorrect as these MFR limits reflect the
rated limit as opposed to a limit which will generate warning. Instead
return -ENXIO like we were already doing for other WARN_LIMITs.

Subsequently these rated limits have been exposed generically as new
fields in the sysfs ABI so the values are still available.

Fixes: 15b2703e5e02 ("hwmon: (pmbus) Add driver for BluTek BPA-RS600")
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20210812014000.26293-2-chris.packham@alliedtelesis.co.nz
Signed-off-by: Guenter Roeck <linux@roeck-us.net>