NetworkPkg: Replace ASSERT with error return code in PXE and HTTP boot driver.

NetworkPkg: Replace ASSERT with error return code in PXE and HTTP boot driver.

This patch remove the ASSERT when receive a DHCP packet large than the maximum
cache buffer size.

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