This fixes a race between power supply device and initial
attributes creation, plus makes it possible to implement
writable properties.

[Daniel Mack - removed superflous return statement
 and dropped .mode attribute from POWER_SUPPLY_ATTR]
Suggested-by: NGreg KH <gregkh@suse.de>
Suggested-by: NKay Sievers <kay.sievers@vrfy.org>
Signed-off-by: NAnton Vorontsov <cbouatmailru@gmail.com>
Tested-by: NDaniel Mack <daniel@caiaq.de>