net: Fix unused variable compile warning

If CONFIG_BOOTP_SERVERIP is not defined, unused variable warning is
reported.  This was fixed upstream using a compiler feature instead
of a simple reorder of the statements.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>