KVM: PPC: Book3S: Eliminate some unnecessary checks

In an excess of caution, commit 6f63e81b ("KVM: PPC: Book3S: Add
MMIO emulation for FP and VSX instructions", 2017-02-21) included
checks for the case that vcpu->arch.mmio_vsx_copy_nums is less than
zero, even though its type is u8.  This causes a Coverity warning,
so we remove the check for < 0.  We also adjust the associated
comment to be more accurate ("4 or less" rather than "less than 4").

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>