ppc4xx: Fix i2c repeated start

Debugging some i2c trouble I saw on my scope that repeated
start is not working properply. The 4xx even held clock pulled down
after transfers. Having a look in the driver I realized
that IIC_CNTL_RPST is set on that part of the transfer that should
begin with a repeated start. But repeated start is about not sending a
stop condition, so IIC_CNTL_RPST has to be set on the last transfer
before the repeated start happens.

Signed-off-by: Dirk Eibach <dirk.eibach@gdsys.cc>
Reviewed-by: Stefan Roese <sr@denx.de>