GpioSampler broken

[rleh]

At some point, probably while refactoring the system clock stuff, the gpio sampler code broke for some of the STM32 targets (e.g. Nucleo-F411RE).
Since at least release 2021q3 some of the gpio sampler unit tests fail:

>>> gpio_sampler_test␊
Channel 0:␊
-1260099550␊
-1260099321␊
-1259929112␊
-1259758983␊
-1259588858␊
Channel 1:␊
-1260099549␊
-1259418692␊
-1259248537␊
-1259078412␊
-1258908281␊
FAIL: gpio_sampler_test:71 : -229 not in range [0,170000]␊
FAIL: gpio_sampler_test:72 : -170209 not in range [170000,187000]␊
FAIL: gpio_sampler_test:73 : -170129 not in range [170000,187000]␊
FAIL: gpio_sampler_test:74 : -170125 not in range [170000,187000]␊
FAIL: gpio_sampler_test:77 : -680857 not in range [680000,697000]␊
FAIL: gpio_sampler_test:78 : -170155 not in range [170000,187000]␊
FAIL: gpio_sampler_test:79 : -170125 not in range [170000,187000]␊
FAIL: gpio_sampler_test:80 : -170131 not in range [170000,187000]␊