net: mvneta: Use devm_kmalloc_array() in mvneta_init()

* 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_kmalloc_array".

  This issue was detected by using the Coccinelle software.

* Replace the specification of a data type 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>
Signed-off-by: David S. Miller <davem@davemloft.net>