spi: struct spi_device constification

Merge series from Geert Uytterhoeven <geert+renesas@glider.be>:

After noticing new cases of casting away constness, I went over all
spi_get_*() functions and their callers, and made the following changes:
  1. Make all pointer parameters const where possible,
  2. Remove unneeded casts, some not even related to constness.