* SPI: Fix discarded-qalifiers warning when compiling with all warnings
This fixes an error introduced with changeset b847f41 which
tightened the use of const for read-only data. The helper
funtion __transferBytes also requires the const qualifier on
outgoing data. Without this change a warning is displayed
when compiling with the Arduino IDE set to display "All"
compiler warnings.
Tests:
- Build an ESP32 SPI sketch that uses static const data to send
to an SPI device using the SPI.transferBytes() API
* SPI:Ensure all local functions are marked static
This audits all functions in the esp32-hal-xpi.c module and
ensures that any functions entirely local to the module are
marked as static.
Tests:
- Build with Arduino set to show all warnings and ensure none
are displayed
* SPI: Remove unused local __spiTranslate24 function
This removes the __spiTranslate24() function which is unused.
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