wil6210: fix overwriting max_assoc_sta module param

Save the parameter of max_assoc_sta per device.
On fw_ready event calculate the max_assoc_sta and save it per device,
do not overwrite the max_assoc_sta module param.

Signed-off-by: Ahmad Masri <amasri@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>