spi: bitbang: Use typedef for txrx_*() callbacks

With a typedef for the txrx_*() callbacks the code looks neater.
Note that typedef for a function is okay to have.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://msgid.link/r/20240517194104.747328-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>