pwm: fix used-uninitialized warning in pwm_get()

Some versions of GCC don't seem no notice that the initialization of the
index variable is tied to that of the chip variable and falsely report
it as potentially being used uninitialized. However, to save anybody
else from tripping over this, we now initialize the index variable
unconditionally.

Originally-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>