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

include/linux/spi/spi.h

@@ -34,7 +34,7 @@ extern struct bus_type spi_bus_type;
 
 /**
  * struct spi_statistics - statistics for spi transfers
- * @clock:         lock protecting this structure
+ * @lock:          lock protecting this structure
  *
  * @messages:      number of spi-messages handled
  * @transfers:     number of spi_transfers handled