drm/amdgpu: fix return value check in kfd

This patch fixes a return value check in kfd doorbell handling.
This function should return 0(error) only when the ida_simple_get
returns < 0(error), return > 0 is a success case.

Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Fixes: 16f00131 ("drm/amdkfd: Allocate doorbells only when needed")
Acked-by: Christian Koenig <chriatian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_doorbell.c

@@ -280,7 +280,7 @@ phys_addr_t kfd_get_process_doorbells(struct kfd_process_device *pdd)
 	if (!pdd->doorbell_index) {
 		int r = kfd_alloc_process_doorbells(pdd->dev,
 						    &pdd->doorbell_index);
-		if (r)
+		if (r < 0)
 			return 0;
 	}