MIPS: KVM: Convert a fallthrough comment to fallthrough

There is a fallthrough comment being forgotten,
GCC complains about it:

arch/mips/kvm/emulate.c: In function kvm_mips_emulate_load:
arch/mips/kvm/emulate.c:1936:21: error: this statement may fall through
 1936 |   vcpu->mmio_needed = 1; /* unsigned */
      |   ~~~~~~~~~~~~~~~~~~^~~
arch/mips/kvm/emulate.c:1939:2: note: here
 1939 |  case lw_op:

Just fix it.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Huacai Chen <chenhc@lemote.com>
Message-Id: <20200810011749.3211128-1-jiaxun.yang@flygoat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>