crypto: cavium/nitrox - Fix 'nitrox_get_first_device()' when ndevlist is fully iterated

When a list is completely iterated with 'list_for_each_entry(x, ...)', x is
not NULL at the end.

While at it, remove a useless initialization of the ndev variable. It
is overridden by 'list_for_each_entry'.

Fixes: f2663872

 ("crypto: cavium - Register the CNN55XX supported crypto algorithms.")
Cc: <stable@vger.kernel.org>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>