net: uip: Fix uip_udp_conns traverse loop in uip_process

the traverse step length should be sizeof(struct uip_udp_conn)
in uip_process when matching the right uip_udp_conn for net_buf

Change-Id: I33c0499582a96e5031ba94f147ff2376726b3f9c
Signed-off-by: Leyi Rong <leyi.rong@intel.com>