crypto: cavium - clean up clang warning on unread variable offset

The variable offset is being assigned and not being used; it should
be passed as the 2nd argument to call to function nitrox_write_csr
but has been omitted. Fix this.

Cleans up clang warning: Value stored to 'offset' is never read

Fixes: 14fa93cd ("crypto: cavium - Add support for CNN55XX adapters.")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>