net/octeontx: fix user supplied MAC address index

Earlier after a successful mac_addr_add operation, index was returned
by underlying layer which was unused but same as provided by DPDK API.

So API is enhanced to use application provided index location to add
MAC address entry.

Fixes: e4373bf1b3f5 ("net/octeontx: add unicast MAC filter")

Signed-off-by: Sunil Kumar Kori <skori@marvell.com>
Acked-by: Harman Kalra <hkalra@marvell.com>