NetworkPkg/Udp6Dxe: Fix the failure to leave one multicast group address.

NetworkPkg/Udp6Dxe: Fix the failure to leave one multicast group address.

The issue was enrolled by the commit of ceec3638. One of the change in the commit
was to return the status from NetMapIterate in Udp6Groups function. But it should
not return EFI_ABORTED directly in case McastIp is not NULL, which means to terminate
the iteration and leave the McastIp successfully.

Cc: Wang Fan <fan.wang@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>