disas/riscv: Fix 6-bit immediate extraction

rv_codec_r2_imm6 is used for XThead instructions whose 6-bit
immediate field is encoded in bits 25:20. The old expression
left-shifted by 38 and then right-shifted by 60, so it kept only
bits 25:22.

Use extract32() to decode bits 25:20 directly. This fixes the
immediate printed for th.srri and th.tst.

Fixes: 318df7238b9f ("disas/riscv: Add support for XThead* instructions")
Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: TANG Tiancheng <lyndra@linux.alibaba.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20260703-b4-disas-xthead-fix-riscv-next-v4-2-84c566330bc7@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit 7a8454ef293085a45686929577b7d5a5daded242)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>