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.