net: atlantic: loopback configuration in improper place

Initial loopback configuration should be called earlier, before
starting traffic on HW blocks. Otherwise depending on race conditions
it could be kept disabled.

Fixes: ea4b4d7f

 ("net: atlantic: loopback tests via private flags")
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>