pwm: mediatek: Fix Kconfig description

Fix a copy/paste error that sneaked into the Kconfig description of the
Mediatek PWM driver.

Signed-off-by: Zhi Mao <zhi.mao@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>