spi: mpc52xx-psc: Make mpc52xx_psc_spi_transfer_one_message() static

spi: mpc52xx-psc: Make mpc52xx_psc_spi_transfer_one_message() static

With W=1:

    drivers/spi/spi-mpc52xx-psc.c:178:5: warning: no previous prototype for ‘mpc52xx_psc_spi_transfer_one_message’ [-Wmissing-prototypes]
      178 | int mpc52xx_psc_spi_transfer_one_message(struct spi_controller *ctlr,
	  |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

mpc52xx_psc_spi_transfer_one_message() is only used inside the file that
defines it.  Hence fix this by making it static.

Fixes: 145cfc3840e5931a ("spi: mpc52xx-psc: Switch to using core message queue")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310061815.7Rtyi4hs-lkp@intel.com/
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20231006112945.1491265-1-geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>