hwmon: (pmbus/adm1275) Fix power sampling support

Not every chip supported by this driver supports setting the number
of samples for power averaging. Also, the power monitoring register
is not always a 16-bit register, and the configuration bits used for
voltage sampling are different depending on the register width.
Some conditional code is needed to fix the problem.

On top of all that, the compiler complains about problems with
FIELD_GET and FIELD_PREP macros if the file is built with W=1.
Avoid using those macros to silence the warning.

Cc: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Cc: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Reviewed-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>