net: cdc_ncm: error path lock fix

Fixes the sparse warning

 drivers/net/usb/cdc_ncm.c:836:9: warning: context imbalance in 'cdc_ncm_txpath_bh' - different lock contexts for basic block

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>