gigaset: fix device ERROR response handling

Clear out pending command that got rejected with 'ERROR' response.
This fixes the bug where unloading the driver module would hang
with the message: "gigaset: not searching scheduled commands: busy"
after a device communication error.

Impact: error handling bugfix
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>