OMAP: omap_device: track latency in nanoseconds

Rather than having to do a usecs = nsecs / NSECS_PER_USEC to
track latency in usecs, just track it in nanoseconds.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>