power: charger-manager: comment spelling fixes

By accident I stumbled over a few misspelled words in the
charger-manager header file which this patch fixes. Namely:
- Extcon rather than Exton
- constraint rather than constratint
- existence rather than existance
- difference rather than diffential

While at it also add a missing space before a closing comment star
forward-slash.

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>