spi: spi.c: Add missing __percpu annotations in users of spi_statistics

Fixes sparse warnings of this kind:
drivers/spi/spi.c:117:16: sparse:     expected struct spi_statistics *
drivers/spi/spi.c:117:16: sparse:     got struct spi_statistics [noderef]
 __percpu *[assigned] pcpu_stats

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Jander <david@protonic.nl>
Link: https://lore.kernel.org/r/20220805084458.1602277-1-david@protonic.nl
Signed-off-by: Mark Brown <broonie@kernel.org>