net: if: default `NET_IF_LOWER_UP` at compile time

The `net_if` layer should not be forcing `NET_IF_LOWER_UP` to be set on
all interfaces at runtime. Because `net_init` runs relatively late by
default, this is overriding any control the driver itself may have
performed using `net_if_carrier_on/off`.

Initialise this bit at compile-time instead. As the structure is already
being initialised, this doesn't increase any footprints.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>