ath6kl: Add a modparam to enable multi normal interface support

This option lets operate more than one vif in normal mode (AP/STA/IBSS)
when support for multiple vif is enabled. This modparam needs to be used
as

modprobe ath6kl multi_norm_if_support=1

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