swim: fix adapter speed handling

swim: fix adapter speed handling

SWIM transport only supports two adapter speeds:
- "low speed"  equal to 363 kHz (8 MHz / 22)
- "high speed" equal to 800 kHz (8 MHz / 10)

Replace the previous convention that use "0" or "1" for "low" or
"high" speed with the effective speed in kHz.
Rework the implementation of stlink_speed_swim().
Set low speed in the stm8 config files, because only low speed is
permitted at debug connection; the previous code ignores the
initial value.

Change-Id: I2484c9419a2c554c59eb6b9216339393ab0b54f3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5529
Tested-by: jenkins