Add checks to not enable multiple loopback modes simultaneously,
It was also discovered that for dma loopback to function correctly
promisc mode should be enabled on device.

Fixes: ea4b4d7f ("net: atlantic: loopback tests via private flags")
Signed-off-by: Nikita Danilov <ndanilov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: Dmitry Bogdanov <dbogdanov@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>