s390/net: move pnet constants

There is no need to define these PNETID related constants in
the pnet.h file, since they are just used locally within pnet.c.
Just code cleanup, no functional change.

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>