target/riscv: rvv: Replace VSTART_CHECK_EARLY_EXIT in vext_ldst_us

target/riscv: rvv: Replace VSTART_CHECK_EARLY_EXIT in vext_ldst_us

Because the real vl (evl) of vext_ldst_us may be different (e.g.
vlm.v/vsm.v/etc.), so the VSTART_CHECK_EARLY_EXIT checking function
should be replaced by checking evl in vext_ldst_us.

Signed-off-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20240918171412.150107-3-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>