spi: s3c64xx: fix compiler warning in spi-s3c64xx

The Exynos 7 arm64 support now allows the S3C64xx SPI driver to be
compiled into an ARM64 kernel, so the cast from the [rt]x_dmach int
variable to a void* in this driver now triggers a warning.
Add a long cast to silence the compiler.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>