spi: Add brackets and tidy defines in spi.h

Some of the #defines in spi.h are not bracketed. To avoid future mistakes
add brackets. Also add an explanatory comment for SPI_CONN_DUAL_...

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
1 file changed