ath11k: Remove some duplicate code

'ATH11K_HE_MCS_MAX' is 11, so these 2 blocks of code are exactly the same.
Remove the one that uses a hard-coded constant.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/a65952db7f4eb8aaaa654b77dcd4930482f5c49b.1624483438.git.christophe.jaillet@wanadoo.fr