spi: Remove repeated struct declaration

struct spi_transfer is declared twice. One is declared at 24th line.
The blew one is not needed though. Remove the duplicate.
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20210401065904.994121-1-wanjiabing@vivo.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/linux/spi/spi.h

@@ -247,7 +247,6 @@ static inline void *spi_get_drvdata(struct spi_device *spi)
 }
 
 struct spi_message;
-struct spi_transfer;
 
 /**
  * struct spi_driver - Host side "protocol" driver