unicore32: pwm: Use managed resource allocations

This commit uses the managed resource allocation functions to simplify
the cleanup paths on error and removal.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Qin Rui <qinrui@mprc.pku.edu.cn>
Acked-by: Guan Xuetao <gxt@mprc.pku.edu.cn>