habanalabs: fix race between soft reset and heartbeat

There is a scenario where an ongoing soft reset would race with an
ongoing heartbeat routine, eventually causing heartbeat to fail and
thus to escalate into a hard reset.

With this fix, soft-reset procedure will disable heartbeat CPU messages
and flush the (ongoing) current one before continuing with reset code.

Signed-off-by: Koby Elbaz <kelbaz@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>