NetworkPkg: Fix issue in dns driver when building DHCP packet.

NetworkPkg: Fix issue in dns driver when building DHCP packet.

Currently, DNS driver configure the dhcp message type to inform
when building dhcp packet to get dns info from, but it not works
with dhcp server deployed on linux system. However it works well
when changed to request type.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Wu Jiaxin <jiaxin.wu@intel.com>