mgmt: ec_host_cmd: verify a command before passing it to handler

Verify validity of a received command before passing it to the general
handler.

It allows performing some actions, right after receiving the command.
The context switch is not needed. Such feature may be needed for
overloaded system, where instant reboot is required.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>