net: Make sure IPaddr_t is 32 bits in size

When building u-boot as 64 bit application (e.g. sandbox) ulong might be
64 bits in size. This breaks network code as IPaddr_t is 64 bytes in
size then and an IPv4 address is 32 bits in size. This patch makes sure
that IPaddr_t is always 32 bits in size. Also some warnings introduced
by this patch are fixed.

Signed-off-by: Matthias Weisser <weisserm@arcor.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>