s390/qeth: Use constant for IP address buffers

Use INET6_ADDRSTRLEN constant with size of 48 which be used for char arrays
storing ip addresses (for IPv4 and IPv6)

Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Thorsten Winkler <twinkler@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>