wifi: ath9k: Remove unused variable mismatch

Variable mismatch is just being incremented and it's never used anywhere
else. The variable and the increment are redundant so remove it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20221104140723.226857-1-colin.i.king@gmail.com