USB: serial: sierra driver interrupt urb resubmit fix

- Fixed a problem when re-submitting urb from interrupt callback in
  function sierra_instat_callback(). This suppresses also issuing of
  error messages in /var/log/kern.log
- Removed redundant debug message at the beginning of
  sierra_instat_callback() function
- Changed a debug message to be an error message

Signed-off-by: Elina Pasheva <epasheva@sierrawireless.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>