[NIOS2] Fix disassembly of br.n instruction.

[NIOS2] Fix disassembly of br.n instruction.

The code was checking wrong bit for sign extension. It caused it
to zero-extend instead of sign-extend the immediate value.

2021-03-25  Abid Qadeer  <abidh@codesourcery.com>

	opcodes/
	* nios2-dis.c (nios2_print_insn_arg): Fix sign extension of
	immediate in br.n instruction.

	gas/
	* testsuite/gas/nios2/brn.s: New.
	* testsuite/gas/nios2/brn.d: New.