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>