net: doc: fix compile warning of no format arguments in ifenslave.c

Fix following warning in ifenslave.c with gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4).

Documentation/networking/ifenslave.c:263:4: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:271:3: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:277:3: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:285:3: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:291:3: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:292:3: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:312:4: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:323:3: warning: format not a string literal and no format arguments
Documentation/networking/ifenslave.c:342:4: warning: format not a string literal and no format arguments


Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>