EmbeddedPkg/NonCoherentDmaLib: Avoid dereferencing unset Map field
Map->Operation is used to select whether a DMA region that
is being bounced has the source buffer copied to it. Except
Map->Operation isn't yet set, so the behavior is somewhat
random. Instead use the passed in Operation parameter.
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>