pwm: ab8500: Fix wrong value shift for disable/enable PWM

Current code only works when pdev->id is 1. Fix it by passing correct
bit values to abx500_mask_and_set_register_interruptible().

Having DISABLE_PWM/ENABLE_PWM does not make the code more readable
because the bit values depend on pdev->id. Thus drop the DISABLE_PWM
and ENABLE_PWM defines.

This patch also removes an unnecessary return in ab8500_pwm_disable().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Alexandre BOURDIOL <alexandre.bourdiol@st.com>
Acked-by: Philippe Begnic <philippe.begnic@st.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>