These warnings are caused because frequencies are stored as `uint32_t`,
which is `unsigned long`, while the printf format expects `int`. In
practice, this does not actually cause problems, since on STM32 gcc
`long` and `int` are both 32-bits and frequencies are never large enough
to cause signed vs unsigned ambiguity.
Since printf has no format specifiers for e.g. uint32_t (libc does have
some macros for this, but those really hurt readability), this is tricky
to fix in a portable way (other architectures or compilers might have
`uint32_t` equal to `unsigned int` instead of `unsigned long`), this fix
just casts the frequency to `unsigned` before passing it to printf (and
for good measure, also convert the specifier from `%d` to `%u`). This
does mean this printing will break if `int` is not at least 32-bits
(e.g. on AVR), but given the scope of this library, that should be
acceptable.
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