drm/ttm: fix return value check

The function returns the number of swapped pages here. Only abort when we get
a negative error code.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210409130113.1459-1-christian.koenig@amd.com