selftests/powerpc: Fix usage message in context_switch

When we inverted the behaviour of the flags we forgot to update the
usage message.

Fixes: 51c21e72 ("selftests/powerpc: Make context_switch touch FP/altivec/vector by default")
Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>