USB: serial: kobil_sct: switch 4 remaining printk() calls to use dev_dbg

These somehow got missed previously (as they weren't calling dbg(), but
rather printk() directly), so move over to using dev_dbg() as we never
want to see startup messages unless debugging is enabled.

Cc: Johan Hovold <jhovold@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>