spi: core: Use %zu for printing 'size_t' type

Use %zu for printing 'size_t' type in order to fix the following
build warning on ARM64:

drivers/spi/spi.c: In function '__spi_split_transfer_maxsize':
drivers/spi/spi.c:2278:2: warning: format '%i' expects argument of type 'int', but argument 4 has type 'size_t' [-Wformat=]

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>