spi: core: Staticize __spi_split_transfer_maxsize()

__spi_split_transfer_maxsize() can be made static as it is only
used in this file.

This fixes the following sparse warning:

drivers/spi/spi.c:2266:5: warning: symbol '__spi_split_transfer_maxsize' was not declared. Should it be static?
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2241,7 +2241,7 @@ struct spi_replaced_transfers *spi_replace_transfers(
 }
 EXPORT_SYMBOL_GPL(spi_replace_transfers);
 
-int __spi_split_transfer_maxsize(struct spi_master *master,
+static int __spi_split_transfer_maxsize(struct spi_master *master,
 					struct spi_message *msg,
 					struct spi_transfer **xferp,
 					size_t maxsize,