stlink: fix printed version for new STM8 nucleo board

stlink: fix printed version for new STM8 nucleo board

The ST-LINK/V2.1 embedded in the new nucleo boards for STM8 does
not follow the normal versioning rules, and puts mass-storage and
swim version in the field normally used respectively for jtag and
mass-storage version.

Apply the check suggested by STMicroelectronics to discriminate
the two cases and print the correct version.

Change-Id: I0dd1da11013be3f1e56084489e28cfba98bb07af
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4716
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: Spencer Oliver <spen@spen-soft.co.uk>