disas/riscv: Fix slli_uw decoding

disas/riscv: Fix slli_uw decoding

The decoding of the slli_uw currently contains decoding
error: shamt part of opcode has six bits, not five.

Fixes 3de1fb71("target/riscv: update disas.c for xnor/orn/andn and slli.uw")

Signed-off-by: Ivan Klokov <ivan.klokov@syntacore.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230227090228.17117-1-ivan.klokov@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>