MdeModulePkg/SNP: remove redundant DEBUG print in SNP Transmit.c

MdeModulePkg/SNP: remove redundant DEBUG print in SNP Transmit.c

This patch is to remove some redundant DEBUG output in SNP transmit function.
In case of return EFI_NOT_READY in PxeTransmit, the SNP driver is indicate
the caller that the transmit queue is full, it's a very common situation druing
transmit, not a critical error. So the patch move the DEBUG lever to EFI_D_NET.

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