LoongArch: KVM: Add returns to SIMD stubs

The stubs for kvm_own/lsx()/kvm_own_lasx() when CONFIG_CPU_HAS_LSX or
CONFIG_CPU_HAS_LASX is not defined should have a return value since they
return an int, so add "return -EINVAL;" to the stubs.
Fixes the build error:

In file included from ../arch/loongarch/include/asm/kvm_csr.h:12,
                 from ../arch/loongarch/kvm/interrupt.c:8:
../arch/loongarch/include/asm/kvm_vcpu.h: In function 'kvm_own_lasx':
../arch/loongarch/include/asm/kvm_vcpu.h:73:39: error: no return statement in function returning non-void [-Werror=return-type]
   73 | static inline int kvm_own_lasx(struct kvm_vcpu *vcpu) { }

Fixes: db1ecca2 ("LoongArch: KVM: Add LSX (128bit SIMD) support")
Fixes: 118e10cd

 ("LoongArch: KVM: Add LASX (256bit SIMD) support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>