spi: Fix the spi configuration mode documentation

Reported by Ramesh Thomas, the loop mode is included in the mode
configuration and not as a standalone bit. Also rewording word_size
parameter documentation.

Change-Id: I1e3337087316f56d58e843278b9c65e9f4f5476f
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>