Fixed writing to OTP registers smaller than 32 bits

[Wassasin]

We have a single device-driver definition of the OTP fuse registers, but can access them both from shadow registers, as well as from the OTP ROM API. In the OTP ROM API implementation writing to registers smaller than 32 bits did not do anything. See #25.

This is due to calling chunks_exact with a size of 4, which does not yield any chunk if the remaining buffer cannot fit exactly that chunk:

ec-slimloader/libs/imxrt-rom/src/registers.rs

Lines 114 to 119 in 76d6f5c

	for (i, chunk) in data.chunks_exact(4).enumerate() {
	// Safety: we have chunks of exactly 4 bytes, hence the conversion to [u8; 4] is safe.
	let word = u32::from_le_bytes(unsafe { chunk.try_into().unwrap_unchecked() });
	self.otp.write_fuse(address + i as u32, word, self.mode_locked)?;
	}

This PR streamlines how the OTP words are accessed between OTP ROM API and shadow registers, and adds a few tests to validate that it does so correctly.