crypto: axis - hide an unused variable

Without CONFIG_DEBUG_FS, we get a harmless warning:

drivers/crypto/axis/artpec6_crypto.c:352:23: error: 'dbgfs_root' defined but not used [-Werror=unused-variable]

This moves it into the #ifdef that hides the only user.

Fixes: a21eb94f

 ("crypto: axis - add ARTPEC-6/7 crypto accelerator driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Lars Persson <larper@axis.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>