usb_wwan: fix error in marking device busy

This fixes two errors:
- the device is busy if a message was recieved even if resubmission fails
- the device is not busy if resubmission fails due to -EPERM

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>