pwm: atmel-tcb: Fix typo in comment

Spelling mistake (triple letters) in comment. Detected with the help of
Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>