fix:
should do right shift on wb before clearing

cleanups:
1,should memset all wb buffer
2,set max wb number to 128 (total 4KB) is big enough
Signed-off-by: NMonk Liu <Monk.Liu@amd.com>
Acked-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>