Explicitly handle `WriteCountCheck::OnceOnly`

[JuliDi]

WriteCountCheck::OnceOnly is not handled directly, but only in the "catchall"

mock-flash/src/lib.rs

Lines 214 to 230 in b59a7e4

	*word_writable = match (*word_writable, self.write_count_check) {
	(v, WriteCountCheck::Disabled) => v,
	(Writable::T, _) => Writable::O,
	(Writable::O, WriteCountCheck::Twice) => Writable::N,
	(Writable::O, WriteCountCheck::TwiceDifferent)
	if source_word == &self.data[address..][..BYTES_PER_WORD] =>
	{
	Writable::O
	}
	(Writable::O, WriteCountCheck::TwiceDifferent) => Writable::N,
	(Writable::O, WriteCountCheck::TwiceWithZero)
	if source_word.iter().all(|b| *b == 0) =>
	{
	Writable::N
	}
	_ => return Err(MockFlashError::NotWritable(address as u32)),
	};

This seems not ideal:
When working with flash that uses ECC, writing twice is not desirable because it disables/breaks ECC.
It is (usually?) not invalid to do so, but a more precise error message like "writing more than once is invalid on this flash" might help with troubleshooting.

[JuliDi]

Maybe it would be better to initialize the writable Vector with O nce instead of Twice for this case...