pm: device_runtime: Simplify runtime_enable

Move the check of PM_DEVICE_FLAG_RUNTIME_ENABLED to the beginning of
the function. With this we avoid taking/release the semaphore and also
we no longer need check it in runtime_enable_sync(), because it was
already checked in pm_device_runtime_enable().

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>