net: wan: dlci: Remove set but not used variable 'err'

Fixes the following W=1 kernel build warning(s):

 drivers/net/wan/dlci.c: In function ‘dlci_close’:
 drivers/net/wan/dlci.c:298:8: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Mike McLagan <mike.mclagan@linux.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>