stlink: simplify api version handling

stlink: simplify api version handling

The API version is hardcoded in the STLINK device and depends on
the specific firmware version. There is no way to switch between
different API. Thus, it make no sense to keep in two separate
variables the current and the max API supported because they are
always equal to "the only" API supported by the specific STLINK.

Collapse h->version.jtag_api_max and h->jtag_api in a single
variable h->version.jtag_api
Remove an extra print of api version; this is already advertised
while printing the STLINK version.

Change-Id: I0519694a34748e5a0aa330932c1256080a6d5f20
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4712
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>