net: keystone_net: move serdes setup to initialization function

On Keystone2 devices serdes must be initialized before accessing MDIO bus.
This commit moves the keystone2_net_serdes_setup() from keystone2_eth_open
to keystone2_emac_initialize to meet that requirement.

This also eliminates unnecessary serdes initializatin every time when the
keystone2_eth_open is being called.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Tested-by: Nishanth Menon <nm@ti.com>