i2c-pxa: Clean transaction stop

It was reported to me that the i2c-pxa driver was not able to process
more that 50 transactions per second. Investigation revealed that the
I2C unit was busy for 20 ms after every transaction. The reason seems
to be that we forget to clear the STOP and ACKNACK bits at the end of
the transaction. According to the PXA27x developer's manual, we shall
do so.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Lennert Buytenhek <kernel@wantstofly.org>
Cc: Nicolas Pitre <nico@cam.org>