drivers/net/wan: fix sparse warning: symbol shadows an earlier one

Impact: Rename function scope variable and while being at it fix some
coding style issues.

Fix this sparse warning:
  drivers/net/wan/sbni.c:1526:14: warning: symbol 'num' shadows an earlier one
  drivers/net/wan/sbni.c:160:13: originally declared here

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