MdeModulePkg/ScsiDiskDxe: Fix the build warning/error "unreachable statement"

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16190 6f19259b-4bc3-4df7-8a09-765794883524