objdump: fix extended (256) disassembler colors

After commit:

  commit a88c79b77036e4778e70d62081c3cfd1044bb8e3
  Date:   Tue Aug 9 14:57:48 2022 +0100

      Default to enabling colored disassembly if output is to a terminal.

The 256 extended-color support for --disassembler-color was broken.
This is fixed in this commit.

	PR 29457
	* objdump (objdump_styled_sprintf): Check disassembler_color
	against an enum value, don't treat it as a bool.