[Why]
For debugging underflow issues it can be useful to have asserts when the
underflow initially occurs.

[How]
Read the underflow status registers after actions that have a high risk
of causing underflow and assert that no underflow occurred. If underflow
occurred, clear the bit.
Signed-off-by: NThomas Lim <Thomas.Lim@amd.com>
Reviewed-by: NEric Yang <eric.yang2@amd.com>
Acked-by: NBhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>