ath6kl: Move initialization/deinitialization of scan_table to appropriate functions

By having scan_table in struct ath6kl, it makes sense to move initialization
to ath6kl_init() and deinitialization to ath6kl_destroy().

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