MdeModulePkg: Fix SNP.Initialize() spec conformance issue

MdeModulePkg: Fix SNP.Initialize() spec conformance issue

v2:
*Refine the coding style according edk2 community's feedback.

Current SNP UNDI Initialize command does not follow the UEFI Spec
to update the SNP MediaPresent field. The result for the Initialize
command execution check should be:
StatFlags: (1) Monitor the upper two bits (14 & 15) in the field to know
whether the command has been executed by the UNDI (Not started, Queued,
Error, Complete). (2) Check the other field to see if there is an active
connection to this network device (used to update MediaPresent).
StatCode: After command execution completes, either successfully or not,
this field contains the result of the command execution (success or failure).
This patch is used to fix it.

NOTE: If any UNDI driver does not follow the UEFI Spec for the media status
update, it may meet failure with this more conditions check (StatFlags).

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