The ornamental design for a controller for handpiece, as shown and described.
FIG. 1 is a front perspective view of a controller for handpiece showing my design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken line shown in the drawings is included for the purpose of illustrating portions of the controller for handpiece and forms no part of the claimed design.