spi: fsl-cpm: substitute empty_zero_page with helper ZERO_PAGE(0)

Not all zero page implementations use empty_zero_page global pointer so
let's substitute empty_zero_page occurence with helper ZERO_PAGE(0).

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Link: https://lore.kernel.org/r/20221018215755.33566-2-giulio.benetti@benettiengineering.com


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