spi: spidev_test: Remove the whole "include" directory when make clean

In the current code, it only removes "include/linux/spi/spidev.h" file
when make clean and there still exists useless "include/linux/spi/"
directory, just remove it.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/1581567368-8055-4-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Mark Brown <broonie@kernel.org>