power-supply: Mark 'if' blocks in power_supply_changed_work() with 'likely'

The 'if' statements in power_supply_changed_work() are mostly there for taking
care of races and normally they will always evaluate to true. Optimize them for
fast execution with 'likely' statements.

Also there is need to have better comments in code to mention about the races
clearly. Get them in place.

Cc: Zoran Markovic <zrn.markovic@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>