MIPS/BFD: Use `bfd_size_type' for dynamic symbol table indices

Use the `bfd_size_type' data type for dynamic symbol table indices in
the MIPS backend, in line with generic code and removing the need to use
a cast.

	bfd/
	* elfxx-mips.c (mips_elf_hash_sort_data): Convert the
	`min_got_dynindx', `max_unref_got_dynindx' and
	`max_non_got_dynindx' members to the `bfd_size_type' data type.
	(mips_elf_sort_hash_table): Adjust accordingly.