drivers/net/wan: fix sparse warnings: make do-while a compound statement

Fix this sparse warnings:

  drivers/net/wan/wanxl.c:414:3: warning: do-while statement is not a compound statement
  drivers/net/wan/wanxl.c:441:3: warning: do-while statement is not a compound statement

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