This improves slightly. It also is some groundwork for better DMA
support in TinyGo in the future.
I'm not entirely sure why it improves performance (in theory the old
code should already saturate the SPI bus) but it does, so 🤷
As discussed on Slack, I believe this property does more harm than good:
* I don't think it's used anywhere. None of the drivers use it.
* It is not fully implemented. While values <= 8 might work fine,
values larger than 8 result in extra zero bits (instead of anything
sensible).
* Worse, it doesn't return an error when it's out of range. This is
not an optional property: if the SPI peripheral doesn't support a
particular number of bits, it should return an error instead of
silently limiting the number of bits. This will be confusing to
users.
Therefore, I propose we drop it. Maybe there are good uses for it
(perhaps for displays that use big endian 16-bit values?), but without a
good use case like a driver in tinygo.org/x/drivers, I think it's more
trouble than it's worth.
This makes the code a bit cleaner because ErrTxInvalidSliceSize isn't
redefined in every file that uses SPI and Mode0/Mode1/Mode2/Mode3 is
defined for every target that uses SPI.
Go 1.19 started reformatting code in a way that makes it more obvious
how it will be rendered on pkg.go.dev. It gets it almost right, but not
entirely. Therefore, I had to modify some of the comments so that they
are formatted correctly.
Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
