net: lwip: fix initialization sequence before a command

net: lwip: fix initialization sequence before a command

The things that are done prior to executing a network command with
NET_LWIP are not consistent with what is done with NET. It impacts the
selection of the current device, and more precisely if the active device
is invalid NET would return an error while NET_LWIP would try to pick a
new device. This incorrect behavior was detected thanks to the eth_rotate
sandbox test (dm_test_eth_rotate()).

Fix it by re-using a sequence similar to what NET has in net_loop().
This piece of code is inserted in a function called net_lwip_eth_start()
renamed from net_lwip_eth_set_current().

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>