spi: aspeed: Use devm_iounmap() to unmap devm_ioremap() memory

spi: aspeed: Use devm_iounmap() to unmap devm_ioremap() memory

The AHB IO memory for each chip select is mapped using
devm_ioremap(), so it should be unmapped using devm_iounmap()
to ensure proper device-managed resource cleanup.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510292356.JnTUBxCl-lkp@intel.com/
Signed-off-by: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://patch.msgid.link/20251105084952.1063489-1-chin-ting_kuo@aspeedtech.com
Signed-off-by: Mark Brown <broonie@kernel.org>