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>