hwrng: n2 - Use devm_kcalloc() in n2rng_probe()

* A multiplication for the size determination of a memory allocation
  indicated that an array data structure should be processed.
  Thus use the corresponding function "devm_kcalloc".

* Replace the specification of a data structure by a pointer dereference
  to make the corresponding size determination a bit safer according to
  the Linux coding style convention.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>