crypto: dh - fix memleak in setkey

commit ee34e2644a78e2561742bea8c4bdcf83cabf90a7 upstream.

setkey can be called multiple times during the existence
of the transformation object. In case of multiple setkey calls,
the old key was not freed and we leaked memory.
Free the old MPI key if any.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>