commit e8c214d2 upstream.

We must mask out the overflow bit as well, otherwise
the wptr will never match the rptr again and the interrupt
handler will loop forever.
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>