must set minor_update.enable before write smaller value
to wptr/doorbell, so for sriov we need set that register
bit in hw_init period.

this could fix the SDMA ring test fail after guest reboot

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>