MdeModulePkg: Correct one return status code in SNP Transmit function.

MdeModulePkg: Correct one return status code in SNP Transmit function.

According to UEFI spec, a BUFFER_FULL return status in UNDI Transmit command
indicates UNDI "Transmit buffer is full. Call Get Status command toempty
buffer." So this patch updates the SNP Transmit to return EFI_NOT_READY for
BUFFER_FULL to let the call know the buffer full status.

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>