Fix getting error message when successfully using stlinkV2

When using stlinkV2 you are constantly getting error messages from
'stlink_v1_open' when all goes well and 'stlink_open_usb' is used
instead. This patch fixes this confusing behaviour and makes stlink-sg.c
use logging macros consistently by removing last 'stderr' printout. It
is not needed as 'stlink_v1_open_inner' is verbose enough.