OMAP: omap_device: fix nsec/usec conversion in latency calculations

Use

   usecs = nsecs / NSEC_PER_USEC;

instead of

   usecs = nsecs * NSEC_PER_USEC;

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