linux-cp: fix setting mtu on hardware interfaces

linux-cp: fix setting mtu on hardware interfaces

Type: fix
Fixes: 616447c39231

In lcp_router_link_mtu(), either vnet_hw_interface_set_mtu() or
vnet_sw_interface_set_mtu() is called, based on whether this appears to
be a physical interface. The test to determine whether this is true was
incorrect and probably never worked right so vnet_sw_interface_set_mtu()
was always being called. This causes some breakage with Recent changes
to code which manages interface MTUs. Fix the test so the right function
is called.

Change-Id: I1ecccbce37d5a1e53b2349ed40f3d0d27eb03569
Signed-off-by: Matthew Smith <mgsmith@netgate.com>