wil6210: fix device ready detection

Adjust driver behavior during FW boot. Proper sequence of
events after reset and FW download, is as following:

- FW prepares mailbox structure and reports IRQ "FW_READY"
- driver caches mailbox registers, marks mailbox readiness
- FW sends WMI_FW_READY event, ignore it
- FW sends WMI_READY event with some data
- driver stores relevant data marks FW is operational

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