MIPS/BFD: Streamline hash table references in `mips_elf_sort_hash_table'

Make all hash table references throughout `mips_elf_sort_hash_table' use
`htab', simplifying code and improving readability.

	bfd/
	* elfxx-mips.c (mips_elf_sort_hash_table): Use `htab' throughout
	to access the hash table.