spi: loongson: Fix build warnings about export.h

spi: loongson: Fix build warnings about export.h

After commit a934a57a42f64a4 ("scripts/misc-check: check missing #include
<linux/export.h> when W=1") and 7d95680d64ac8e836c ("scripts/misc-check:
check unnecessary #include <linux/export.h> when W=1"), we get some build
warnings with W=1:

drivers/spi/spi-loongson-core.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing

So fix these build warnings for SPI/Loongson.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://patch.msgid.link/20250608142939.172108-1-chenhuacai@loongson.cn
Signed-off-by: Mark Brown <broonie@kernel.org>