i2c-omap: Set latency requirements only once for several messages

Ordinary I2C read consist of two messages. First a write operation
to tell register address and then read operation to get data.
CPU wake up latency is set and removed twice in read case.
Set latency requirement before the message processing loop
and remove the requirement after the loop to remove latency
adjustment operations between the messages.

Signed-off-by: Samu Onkalo <samu.p.onkalo@nokia.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>