ArmPkg: Fix Ecc error 3002 in StandaloneMmMmuLib
    
    This patch fixes the following Ecc reported error:
    Non-Boolean comparisons should use a compare operator
    (==, !=, >, < >=, <=)
    
    Cc: Bret Barkelew <bret.barkelew@microsoft.com>
    Cc: Sean Brogan <sean.brogan@microsoft.com>
    Cc: Leif Lindholm <leif@nuviainc.com>
    Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
    Cc: Sami Mujawar <sami.mujawar@arm.com>
    Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
    Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>