mfd: stmpe: PWM on 24xx has altfunc 1

On the STMPE2401 and STMPE2401 altfunction 1 corresponds to the
PWM channels. This oneliner was missing in the case-switch, making
it impossible to enable the PWM channel output.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>