hwmon: (pmbus) Add get_error_flags support to regulator ops

The various PMBus status bits don't all map perfectly to the more
limited set of REGULATOR_ERROR_* flags, but there's a reasonable
number where they correspond well enough.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Link: https://lore.kernel.org/r/20220219000359.19985-1-zev@bewilderbeest.net


[groeck: Added missing locking]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>