spi/bitbang: Unexport spi_bitbang_transfer()

Currently no drivers use the ability to override spi_bitbang_transfer()
and if any started this would make it harder to convert the bitbang code
to use transfer_one_message() so remove the export in order to prevent
anyone starting.

Signed-off-by: Mark Brown <broonie@linaro.org>
diff --git a/drivers/spi/spi-bitbang.c b/drivers/spi/spi-bitbang.c
index a63d7da..495ce0a 100644
--- a/drivers/spi/spi-bitbang.c
+++ b/drivers/spi/spi-bitbang.c
@@ -376,7 +376,7 @@
 /**
  * spi_bitbang_transfer - default submit to transfer queue
  */
-int spi_bitbang_transfer(struct spi_device *spi, struct spi_message *m)
+static int spi_bitbang_transfer(struct spi_device *spi, struct spi_message *m)
 {
 	struct spi_bitbang	*bitbang;
 	unsigned long		flags;
@@ -398,7 +398,6 @@
 
 	return status;
 }
-EXPORT_SYMBOL_GPL(spi_bitbang_transfer);
 
 /*----------------------------------------------------------------------*/