power_supply: Ignore -ENODATA errors when generating uevents

Sometimes a driver can not report a meaningful value for a certain property
and returns -ENODATA.

Currently when generating a uevent and a property return -ENODATA it is
treated as an error an no uevent is generated at all. This is not an
desirable behavior.

This patch adds a special case for -ENODATA and ignores properties which
return this error code when generating the uevent.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>