fix: SPI: remove holes between transfer()

[facchinm]

Fixes #145

[iabdalkader]

Does this actually fix the issue? The lock just prevents other callers from accessing the device, and HOLD_ON_CS just makes it not release CS and not disable the SPI device after the transfer is complete.
I scanned the linked issue and the issue it references, quickly, and as far as I can tell there is no solution other than DMA.
If somehow this does help, I think hold-cs should probably be an option passed via settings (added to API etc..).

[mjs513]

As @KurtE's issue pointed out this seems to be an open issue on Zephyr: zephyrproject-rtos/zephyr#73930

Has anyone looked at that or at @KurtE's solution? Not sure that is really going to resolve the issue

[KurtE]

Thanks @facchinm @iabdalkader @mjs513.

@facchinm - which boards did you try this on?

As I mentioned in the issue #145, in the past I have tried this approach to blanketly turn on the HOLD_ON_CS flag or
equivalent, which helps in some cases, in that at least on some boards, it then avoids the delay before release of CS and the delay after the CS is asserted. However on some boards/devices I ran into issues, where maybe it hung SPI, especially with multiple
devices and/or using the hardware cs pin(s). Some of these experiences may have been IMXRT specific, but I know on the
Teensy, several of our display drivers would not function properly with setting it globally, so our output code had
different helper functions to output with continue and output last byte.

As @mjs513 mentioned - there are some Zephyr open issues on some of this, that appear to have several interrelated issues and or things to try, like:

FIFO - is it enabled? Does it work.

Is DMA enabled?

Async Transfer - appears to speed things up, but ran into limits, like there may be a MAX number of bytes you can do
in a transfer. Also on the callback, can you reissue it with updated source pointer? ...

And my guess is, it may be different for each different processor.

But keeping my fingers crossed that this at least improves it.

Thanks