net: dsa: reorder PHY initialization with MTU setup in slave.c

In dsa_slave_create() there are 2 sections that take rtnl_lock():
MTU change and netdev registration. They are separated by PHY
initialization.

There isn't any strict ordering requirement except for the fact that
netdev registration should be last. Therefore, we can perform the MTU
change a bit later, after the PHY setup. A future change will then be
able to merge the two rtnl_lock sections into one.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>