ath6kl: Change ielen in ath6kl_add_new_sta() from u8 to size_t

Otherwise if (ielen <= ATH6KL_MAX_IE) is dead code. It looks safe to
change the type of ielen from u8 to size_t instead of removing this
if check, this ielen can have the length of more than one ies in future.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>