wil6210: fix check for FW responsiveness

"echo" used to be called when no firmware loaded to the NIC,
this causes error output.

Probe firmware with "echo" only after it returned "ready" event.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>