datapth: Suppress error messages on megaflow updates

With subfacets, we'd expect megaflow updates message to carry
the original micro flow. If not, EINVAL is returned and kernel
logs an error message.  Now that the user space subfacet layer is
removed, it is expected that flow updates can arrive with a
micro flow other than the original. Change the return code to
EEXIST and remove the kernel error log message.

Reported-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>