drm/amdgpu: fix unsigned variable instance compared to less than zero

Currenly the error check on variable instance is always false because
it is a uint32_t type and this is never less than zero. Fix this by
making it an int type.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: 7d0e6329 ("drm/amdgpu: update more sdma instances irq support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>