MdePkg/DevicePathFromText: Fix byte orders of iSCSI.Lun

MdePkg/DevicePathFromText: Fix byte orders of iSCSI.Lun

Per UEFI spec, iSCSI.Lun is a 8-byte array with byte #0 in the left.
It means "0102030405060708" should be converted to:
    UINT8[8] = {01, 02, 03, 04, 05, 06, 07, 08}
or  UINT64 = {0807060504030201}

Today's implementation wrongly uses the reversed order.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jie Lin <jie.lin@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>