spi: core: Use min_t(size_t,..)

Use min_t(size_t,..) in order to avoid the following
build warning on ARM64:

include/linux/kernel.h:754:17: warning: comparison of distinct pointer types lacks a cast
  (void) (&_min1 == &_min2);  \
                 ^
drivers/spi/spi.c:2304:17: note: in expansion of macro 'min'
  xfers[0].len = min(maxsize, xfer[0].len);

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