lib: sbi: Refactor the calls to sbi_hart_switch_mode()

Move them into sbi_hsm_hart_start_finish() and sbi_hsm_hart_resume_finish()
to make them easier to manage.

This will be used by subsequent patches.

Suggested-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Evgenii Shatokhin <e.shatokhin@yadro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>