spi: fix kernel-doc warnings in spi.h

Fix the following 'make htmldocs' warnings:

  .//include/linux/spi/spi.h:71: warning: No description found for parameter 'lock'
  .//include/linux/spi/spi.h:71: warning: Excess struct/union/enum/typedef member 'clock' description in 'spi_statistics'

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Mark Brown <broonie@kernel.org>