scmi: Fix race on timed out transmissions

When a waited-for outstanding request is determined to have timed out,
mark such request immediately as such, so as to avoid any possible race on
the IRQ path with code path evaluating that same timed out condition.

Tested on: Arm Morello Board
Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47419
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>