ath6kl: Cleanup fw interface type setting

It is not necessary to use ath6kl_get_fw_iftype() to find out the
firmware interface type during initialization because the type
of the initial interface in INFRA_NETWORK. Hardcode the fw interface
type corresponding to INFRA_BSS instead of using ath6kl_get_fw_iftype().

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