disas/riscv: Fix vsetivli disassembly

disas/riscv: Fix vsetivli disassembly

The first immediate field is unsigned, whereas operand_vimm
extracts a signed value.  There is no need to mask the result
with 'u'; just print the immediate with 'i'.

Fixes: 07f4964d178 ("disas/riscv.c: rvv: Add disas support for vector instructions")
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>