power_supply: Assume mains power by default

If no power class device is found in power_supply_is_system_supplied(),
the function currently returns 0, which basically means that the system
is supposed to be running on battery. In practice, mobile devices tend
to always implement at least one power class device and more often two
(battery and AC adapter). Systems with no registered power class
devices are more likely to be desktop systems, where the system is
always powered by mains.

So, change the default return value of
power_supply_is_system_supplied() from 0 (running on battery) to 1
(running on mains.)

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>