coresight: Use devm_bitmap_zalloc when applicable

'drvdata->chs.guaranteed' is a bitmap. So use 'devm_bitmap_kzalloc()' to
simplify code, improve the semantic and avoid some open-coded arithmetic
in allocator arguments.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/a4b8454f560b70cedf0e4d06275787f08d576ee5.1635964610.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>