s390/net: convert pnetids to ascii

Pnetids are retrieved from the underlying hardware as EBCDIC. This patch
converts pnetids to ASCII.

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