cxl/region: Fix an ERR_PTR() vs NULL bug

cxl/region: Fix an ERR_PTR() vs NULL bug

The __cxl_decoder_detach() function is expected to return NULL on error
but this error path accidentally returns an error pointer.  It could
potentially lead to an error pointer dereference in the caller.  Change
it to return NULL.

Fixes: b3a88225519c ("cxl/region: Consolidate cxl_decoder_kill_region() and cxl_region_detach()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/7def7da0-326a-410d-8c92-718c8963c0a2@sabinyo.mountain
Signed-off-by: Dave Jiang <dave.jiang@intel.com>