[mlir][tosa] Correct tosa.avg_pool2d for specification error

Specification specified the output type for quantized average pool should be
an i32. Only accumulator should be an i32, result type should match the input
type.

Caused in https://reviews.llvm.org/D111590

Reviewed By: sjarus, GMNGeoffrey

Differential Revision: https://reviews.llvm.org/D112484