usbtmc: Add, clarify and fix comments

Add information regarding lifespan of kref protection:
   Clarify comment on kref_get for interrupt in urb in usbtmc_probe()
   Add comment on kref_get in usbtmc_open()

Fix endpoint reference in documentation for send_request_dev_dep_msg_in()

Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>