scmi: Avoid a use-after-free

Use LIST_FOREACH_SAFE to avoid a use-after-free in scmi_reqs_pool_free.
The next pointer will be invalid after the call to free meaning
LIST_FOREACH will dereference a freed struct to move to the next item.

Reviewed by: emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D50753