pwm: Fix possible ZERO_SIZE_PTR pointer dereferencing error.
Since we cannot make sure the 'chip->npwm' will always be none zero here,
and then if either equal to zero, the kzalloc() will return ZERO_SIZE_PTR,
which equals to ((void *)16).
So this patch fix this with just doing the zero check before calling kzalloc().
Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com> Signed-off-by: Thierry Reding <thierry.reding@gmail.com>