RISC-V: KVM: Fix reg_val check in kvm_riscv_vcpu_set_reg_config()

The reg_val check in kvm_riscv_vcpu_set_reg_config() should only
be done for isa config register.

Fixes: 9bfd900b ("RISC-V: KVM: Improve ISA extension by using a bitmap")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>