LoongArch: KVM: Fix missing EMULATE_FAIL in kvm_emu_mmio_read()

LoongArch: KVM: Fix missing EMULATE_FAIL in kvm_emu_mmio_read()

In the ldptr (0x24...0x27) opcode decoding path, the default case only
breaks out but without setting "ret" value to EMULATE_FAIL. This leaves
run->mmio.len uninitialized (stale from a previous MMIO operation) while
"ret" value remains EMULATE_DO_MMIO, causing the code to proceed with an
incorrect MMIO length.

Add "ret = EMULATE_FAIL" to match the other default branches in the same
function (e.g. the 0x28...0x2e and 0x38 cases).

Cc: stable@vger.kernel.org
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>