net: constify net_class

AFAICS all users of net_class take a const struct class * argument.
Therefore fully constify net_class.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>