Replace the NX842_MEM_COMPRESS define with a function that returns the
specific platform driver's required working memory size.

The common nx-842.c driver refuses to load if there is no platform
driver present, so instead of defining an approximate working memory
size that's the maximum approximate size of both platform driver's
size requirements, the platform driver can directly provide its
specific, i.e. sizeof(struct nx842_workmem), size requirements which
the 842-nx crypto compression driver will use.

This saves memory by both reducing the required size of each driver
to the specific sizeof() amount, as well as using the specific loaded
platform driver's required amount, instead of the maximum of both.
Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>