stlink: always use a valid endpoint

stlink: always use a valid endpoint

In order to extend the driver to support stlink-server over TCP,
we should always use a valid endpoint, as stlink-server is not permissive
and do not accept the invalid STLINK_NULL_EP.

STLINK_NULL_EP value was used for commands without an expected reply,
this value could be replaced with a valid endpoint without any impact
when the size is set to zero.

Change-Id: I003ad364e03d3a10bc036772db86310d996cbe81
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5455
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>