pwm-backlight: Use devm_pwm_get() instead of pwm_get()

Managed versions of the pwm_get() and pwm_put() functions were recently
added to the PWM framework. They can be used to simplify cleanup in the
error paths and the device removal callback.

Cc: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>