wil6210: Add Support for Extended DMG MCS 12.1

FW reports Tx/Rx extended MCS 12.1 to driver as "26". Driver will
convert this into base MCS 7 + EXTENDED_SC_DMG flag so kernel can do
the correct phy rate conversion. Also add log prints to print "12.1"
instead of "26" for extended MCS.

Signed-off-by: Max Chen <mxchen@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1609977050-7089-3-git-send-email-mxchen@codeaurora.org