The default driver type is just an index into a device array defined
by the board.
If a platform wants to encode additional information in the device type,
it can define a custom type.
This means we can just set the default type to whatever fits the target
CPU best.
On ARM this will still be a 32 bit word, but on AVR it will by a 8 bit byte.
There is no way to properly handle incorrect SPI parameters at run time, so
just having an assertion blow up is the better choice here.
As most instances of `spi_acquire()` don't check the return value anyway, this
will improve debugging experience quite a bit. Some implementation of
spi_acquire() don't even check parameters anyway.
- Use negative errno as error codes, rather than home-grown enums
- Update the home-grown enum with negative errno codes for backward
compatibility and mark it deprecated
- Update API doc to use negative errno codes
- Fix various style issues in Doxygen doc
- Use `@retval` to document specific return values instead of abusing
`@return` for this
- Align parameters to proper indent level
- unify file/module @brief
- distinct @file section more visibly in source
- provide missing @brief's
- move module details/notes from file to module sections
- provide missing macro definition for building
