pwm: lpc32xx: Remove handling of PWM channels

In the Linux kernel, the following vulnerability has been resolved:

pwm: lpc32xx: Remove handling of PWM channels

Because LPC32xx PWM controllers have only a single output which is registered as the only PWM device/channel per controller, it is known in advance that pwm->hwpwm value is always 0. On basis of this fact simplify the code by removing operations with pwm->hwpwm, there is no controls which require channel number as input.

Even though I wasn't aware at the time when I forward ported that patch, this fixes a null pointer dereference as lpc32xx->chip.pwms is NULL before devm_pwmchip_add() is called.