bonding: check slave set command firstly

This patch is a little improvement. If user use the
command shown as below, we should print the info [1]
instead of [2]. The eth0 exists actually, and it may
confuse user.

$ echo "eth0" > /sys/class/net/bond4/bonding/slaves

[1] "bond4: no command found in slaves file - use +ifname or -ifname"
[2] "write error: No such device"

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>