os-vif currently set the requested mtu as part of separate request
to the port add command. As a result the port is initially created
with mbufer pool for the jumboframe size, which results in an
error in openvswitch-vswitch logs.
This change moves the mtu update request into ovsdb transaction.
Closes-Bug: #1959586
Change-Id: I1acd74efc100c6f949b5c72525c455aebfa2c50e