mpi: Simplify mpi_powm.

* mpi/mpi-pow.c (_gcry_mpi_powm): Simplify the loop.

This fix is not a solution for the problem reported (yet). The
problem is that the current algorithm of _gcry_mpi_powm depends on
exponent and some information leaks is possible.

• Reported-by: Andreas Zankl <andreas.zankl@aisec.fraunhofer.de>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>