regulator: regulator_common: Cleans sleep related preprocesor directives

Removes conditional calling k_busy_wait() on single threaded
systems and uses k_sleep for both single and multi threaded
systems.

Commit e375d82 ("kernel: Implement k_sleep for Single Thread")
eliminated the need to call k_busy_wait() on single threaded
systems in place of k_sleep.

Signed-off-by: Ricardo Rivera-Matos <rriveram@opensource.cirrus.com>