rsi: optimize virtual interfaces

Due to multiple calls of add interface routine, vif is getting duplicated
and at certain instance, we are out of vifs, causing the driver to behave
abnormal.

Fix: Every vif has a unique mac-id, when we got a vif with same mac-id as
the previous id's, we will override the respective vif.

Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Sushant Kumar Mishra <sushant.mishra@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>