[mlir] Fix warnings

This patch fixes:

  mlir/lib/Dialect/Tosa/IR/TosaOps.cpp:866:35: error: comparison of
  integers of different signs: 'int64_t' (aka 'long') and 'size_t'
  (aka 'unsigned long') [-Werror,-Wsign-compare]

  mlir/lib/Dialect/Tosa/IR/TosaOps.cpp:888:29: error: comparison of
  integers of different signs: 'int64_t' (aka 'long') and 'size_t'
  (aka 'unsigned long') [-Werror,-Wsign-compare]

  mlir/lib/Dialect/Tosa/IR/TosaOps.cpp:893:59: error: comparison of
  integers of different signs: 'int64_t' (aka 'long') and 'size_t'
  (aka 'unsigned long') [-Werror,-Wsign-compare]