wil6210: update WIL_MCS_MAX to 15

Update max MCS to 15, which is supported by Talyn-MB.
This will allow collecting statistics on number of RX packets
in higher MCS.

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