pwm: core: Fix coding style issues

Fixes the following:
WARNING: line over 80 characters
ERROR: spaces required around that ':' (ctx:VxW)
WARNING: Prefer pr_warn(... to pr_warning(...

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>