ath6kl: remove redundant variable ies_len

To get rid of W=1 warning: variable ‘ies_len’ set but not used.
Variable ies_len is being assigned but never read.

Signed-off-by: Kenneth Lu <kuohsianglu@gmail.com>
Reviewed-by: Steve deRosier <derosier@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>