pwm: stmpe: Fix wrong register offset for hwpwm=2 case

Fix trivial copy/paste bug.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Fixes: ef1f09ec

 ("pwm: Add a driver for the STMPE PWM")
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>