tgsi: fix mixed data type comparison in tgsi_point_sprite.c

tgsi: fix mixed data type comparison in tgsi_point_sprite.c

Cast the unsigned semantic index to integer datatype before comparing
to max_generic, otherwise, max_generic which is initialized to -1
will be converted to unsigned int before the comparison, causing a wrong
semantic index to be assigned to a shader output.

Fixes the assert running TurboCAD_gl.trace. (VMware bug 1667265)

Also tested with glretrace, mesa demos pointblast, spriteblast and pointcoord.

v2: use the original max_generic variable but add the (int) cast
    to the semantic index, as suggested by Brian.

Reviewed-by: Brian Paul <brianp@vmware.com>