hwmon: (pmbus) Strengthen check for status register existence

With virtual register page support, it is now possible that the status register
on virtual pages does not exist or is itself virtual. To take this into account
when creating alarm attributes, generate those attributes only if the status
register on the respective page is known to exist.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>