zephyr: correct shimmed task priorities
zephyr: correct shimmed task priorities

Add build assert to ensure that deferred task is lower than all of the
other shimmed tasks.

Also make priority of tasks from 0 to COUNT - 1 instead of 1 to COUNT

BRANCH=none
BUG=none
TEST=volteer still builds

Signed-off-by: Jett Rink <9d4aef0593e6843318d99df31597fb6153b89787@chromium.org>
Change-Id: Iabe9f1117850975ec9bf3df1dcfc690b4725a7e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2523389
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Reviewed-by: Jack Rosenthal <d3f605bef1867f59845d4ce6e4f83b8dc9e4e0ae@chromium.org>

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