power: supply: ltc2941-battery-gauge: fix use-after-free

This driver's remove path calls cancel_delayed_work().
However, that function does not wait until the work function
finishes. This could mean that the work function is still
running after the driver's remove function has finished,
which would result in a use-after-free.

Fix by calling cancel_delayed_work_sync(), which ensures that
that the work is properly cancelled, no longer running, and
unable to re-schedule itself.

This issue was detected with the help of Coccinelle.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>