spi: s3c64xx: fix NULL-deref on driver unbind

In the Linux kernel, the following vulnerability has been resolved: spi: s3c64xx: fix NULL-deref on driver unbind A change moving DMA channel allocation from probe() back to s3c64xx_spi_prepare_transfer() failed to remove the corresponding deallocation from remove(). Drop the bogus DMA channel release from remove() to avoid triggering a NULL-pointer dereference on driver unbind. This issue was flagged by Sashiko when reviewing a controller deregistration fix.