MdeModulePkg/RamDiskDxe: Perform MediaId check first in BlkIo services

MdeModulePkg/RamDiskDxe: Perform MediaId check first in BlkIo services

The commit places the check for MediaId at the beginning of Block IO
services:
RamDiskBlkIoReadBlocks and
RamDiskBlkIoWriteBlocks

This aligns with the Block IO protocol implementations for other
devices.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>