powerpc/eeh: Remove stale CAPI comment

Support for switching CAPI cards into and out of CAPI mode was removed a
while ago. Drop the comment since it's no longer relevant.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190903101605.2890-7-oohall@gmail.com