drivers/net/wan: fix warning: format not a string literal and no ...

Impact: Use 'static const char[]' instead of 'static char[]'.

Fix this compilation warnings:
  drivers/net/wan/lapbether.c: In function 'lapbeth_init_driver':
  drivers/net/wan/lapbether.c:441: warning: format not a string literal and no format arguments
  drivers/net/wan/z85230.c: In function 'z85230_init_driver':
  drivers/net/wan/z85230.c:1782: warning: format not a string literal and no format arguments

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>