drm/amdkfd: Fix permissions of hang_hws

Reading from /sys/kernel/debug/kfd/hang_hws would cause a kernel
oops because we didn't implement a read callback. Set the permission
to write-only to prevent that.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: shaoyunl <shaoyun.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>