MdeModulePkg: Update the default size of MNP TX buffer pool.

MdeModulePkg: Update the default size of MNP TX buffer pool.

The MNP and SNP are independent drivers so the MNP_TX_BUFFER_INCREASEMENT could choose
any value as needed. While this patch update the default value of this macro to 32,
to be same as the maximum recycled buffer numbers in one UNDI GetStatus command. It is
a workaround for some incorrect UNDI drivers which may not return the correct statuses
for the SNP and MNP to correctly retry/call GetStatus to clear the buffers.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Sriram Subramanian <sriram-s@hpe.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>