zephyr: emul: Fix clearing message queue in TCPCI partner
zephyr: emul: Fix clearing message queue in TCPCI partner

SYS_SLIST_CONTAINER evaluate first argument two times, so it cannot be
used with expressions that have side effects. Because of that it is
replaced with CONTEINER_OF to obtain message from list node.

BUG=none
BRANCH=none
TEST=make configure --test zephyr/test/drivers

Signed-off-by: Tomasz Michalec <a547db31280d14f095c760a23a7523d0b1e55fe6@semihalf.com>
Change-Id: Ib67b08468a9ab397a289dfdabc4a1d71f0362536
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3368263
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>
Tested-by: Tomasz Michalec <c064a6042954d4ba0805003dc021b76413ff1548@google.com>
Commit-Queue: Tomasz Michalec <c064a6042954d4ba0805003dc021b76413ff1548@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec