net: wan: sbni: fix device usage count

dev_get_by_name() will increment the usage count if the matching device
is found. But we were not decrementing the count if we have got the
device and the device is non-active.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>