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>