spi: core: Add spi_transfer_is_last() helper

This adds the function spi_transfer_is_last() which can be used by
drivers to know whether a given transfer is the last one in the
current message.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -1049,4 +1049,10 @@ spi_unregister_device(struct spi_device *spi)
 extern const struct spi_device_id *
 spi_get_device_id(const struct spi_device *sdev);
 
+static inline bool
+spi_transfer_is_last(struct spi_master *master, struct spi_transfer *xfer)
+{
+	return list_is_last(&xfer->transfer_list, &master->cur_msg->transfers);
+}
+
 #endif /* __LINUX_SPI_H */