net: qcom/emac: Demote MTU change print to debug

Changing the MTU can be a frequent operation and it is already clear
when (or not) a MTU change is successful, demote prints to debug prints.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Timur Tabi <timur@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>