dhcpv4: simplify dhcpv4_setup_interface()

dhcpv4: simplify dhcpv4_setup_interface()

In the disable case, the function would close the interface uloop fd, free
assignments, check if the interface uloop fd should be closed, and return 0.

Simplify the function by bailing early in the disable case, which also removes
one level of indentation for the gist of the function body.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/266
Signed-off-by: Álvaro Fernández Rojas <[email protected]>