The request flags for acompress is split into two parts.  Part of
it may be set by the user while the other part (ALLOC_OUTPUT) is
managed by the API.

This patch makes the split more explicit by not touching the other
bits at all in the two "set" functions that let the user modify the
flags.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>