soc: intel_adsp: ipc: check for pending ack

This patch modifies intel_adsp_ipc_is_complete function so it don't
report that IPC is completed when we are still waiting for ACK from
HOST.

This change will allow to solve the problem that occurs during the power
state transitions. Occasionally, the Application decides to enter the
power gating state after sending an IPC message, before receiving an ACK
from the HOST. This results in broken IPC communication when coming back
to Idle state.

Signed-off-by: Tomasz Leman <tomasz.m.leman@intel.com>