Fix build warnings when DEBUG_FS is not enabled by using an empty
do-while loop instead of a value:

In file included from ../drivers/crypto/nx/nx.c:27:
../drivers/crypto/nx/nx.c: In function 'nx_register_algs':
../drivers/crypto/nx/nx.h:173:33: warning: statement with no effect [-Wunused-value]
  173 | #define NX_DEBUGFS_INIT(drv)    (0)
../drivers/crypto/nx/nx.c:573:9: note: in expansion of macro 'NX_DEBUGFS_INIT'
  573 |         NX_DEBUGFS_INIT(&nx_driver);
../drivers/crypto/nx/nx.c: In function 'nx_remove':
../drivers/crypto/nx/nx.h:174:33: warning: statement with no effect [-Wunused-value]
  174 | #define NX_DEBUGFS_FINI(drv)    (0)
../drivers/crypto/nx/nx.c:793:17: note: in expansion of macro 'NX_DEBUGFS_FINI'
  793 |                 NX_DEBUGFS_FINI(&nx_driver);

Also, there is no need to build nx_debugfs.o when DEBUG_FS is not
enabled, so change the Makefile to accommodate that.

Fixes: ae0222b7 ("powerpc/crypto: nx driver code supporting nx encryption")
Fixes: aef7b31c ("powerpc/crypto: Build files for the nx device driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Breno Leitão <leitao@debian.org>
Cc: Nayna Jain <nayna@linux.ibm.com>
Cc: Paulo Flabiano Smorigo <pfsmorigo@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-crypto@vger.kernel.org
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>