net: atlantic: better loopback mode handling

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>