arm_tpiu_swo: fix autodetection of SWO pin frequency

arm_tpiu_swo: fix autodetection of SWO pin frequency

While the documentation reports that SWO pin frequency can be
omitted to let the adapter autodetect the value, the code wrongly
drops an error when pin frequency is not specified.

Don't require the pin frequency to be set at "enable", but verify
that the adapter has properly changes it to a valid value.

Change-Id: I3dfbe3256e8887ef4f03512769b06381cdc9db0d
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Karl Palsson <karlp@tweak.net.au>
Fixes: 184724d14e12 ("arm_tpiu_swo: add support for independent TPIU and SWO")
Reviewed-on: https://review.openocd.org/c/openocd/+/6310
Tested-by: jenkins
Reviewed-by: zapb <dev@zapb.de>