drivers/net/wan: fix sparse warning: make symbol static

Fix this sparse warning:

  drivers/net/wan/x25_asy.c:623:5: warning: symbol 'x25_asy_esc' was not declared. Should it be static?

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