MdeModulePkg/AtaAtapiPassThru: don't write read-only AHCI MMIO register

Per AHCI 1.1 spec, AE bit of GHC register is read-only if CAP.SAM is 1

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@16321 6f19259b-4bc3-4df7-8a09-765794883524