wil6210: prevent external wmi commands during suspend flow

In __wmi_send we check if fw is ready at the beginning of the function.
While we wait for the completion of the previous command, system suspend
can be invoked and reset the HW, causing __wmi_send to read from HW
registers while it is not ready.
Taking the wmi_mutex in the reset flow when setting the FW ready bit
to zero will prevent the above race condition.

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>