ArmPkg/ArmDmaLib: consistently use 'gCacheAlignment - 1' as alignment mask

ArmPkg/ArmDmaLib: consistently use 'gCacheAlignment - 1' as alignment mask

We manage to use both an AND operation with 'gCacheAlignment - 1' and a
modulo operation with 'gCacheAlignment' in the same compound if statement.
Since gCacheAlignment is a global of which the compiler cannot guarantee
that it is a power of two, simply use the AND version in both cases.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>