wl18xx: add trigger command and ack event operations

Add the operations that allow wlcore to trigger commands to the
firmware and acknowledge when an event has been fully received.

Allocate a private buffer to hold the maximum sized cmd. Send the
entire length of the buffer each time a command is sent to signal
EOT. Remove the previous EOT mechanism.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>