spi: Fix reference count leak in slave_show()

Fix a reference count leak in slave_show() by properly putting the device
reference obtained from device_find_any_child().

Fixes: 6c364062 ("spi: core: Add support for registering SPI slave controllers")
Fixes: c21b0837 ("spi: Use device_find_any_child() instead of custom approach")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20250319032305.70340-1-linmq006@gmail.com


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