spi: define SPI_XFER_ONCE

The flag combination "SPI_XFER_BEGIN | SPI_XFER_END" is a common use
case of spi_xfer, and it can easily cause an already long line (spi_xfer
takes 5 parameters) to go over the 80 character limit.

define SPI_XFER_ONCE to be a shorter version of the above flag combination.

Cc: Tom Rini <trini@ti.com>
Cc: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>

include/spi.h

@@ -29,6 +29,7 @@
 #define SPI_XFER_END		0x02	/* Deassert CS after transfer */
 #define SPI_XFER_MMAP		0x08	/* Memory Mapped start */
 #define SPI_XFER_MMAP_END	0x10	/* Memory Mapped End */
+#define SPI_XFER_ONCE		(SPI_XFER_BEGIN | SPI_XFER_END)
 
 /* Header byte that marks the start of the message */
 #define SPI_PREAMBLE_END_BYTE	0xec