backport 'regmap: Add missing map->bus check'

Hi,

This commit needs to be backported to 5.4, 5.10, 5.15, it fixes possible null deference from the following commit 'regmap: Add bulk read/write callbacks into regmap_config' which was backported to these kernels in the latest released versions (v5.15.152, v5.10.213, v5.4.272).

Commit 5c422f0b970d287efa864b8390a02face404db5d upstream.

Thanks, MNAdam