opcodes/riscv-dis.c: Tidying with spacing

opcodes/riscv-dis.c: Tidying with spacing

Before changing the core disassembler, we take care of minor code clarity
issues and improve readability.

This commit takes care of improper spacing for code clarity.

opcodes/ChangeLog:

	* riscv-dis.c (riscv_disassemble_insn): Tidying with spacing.