net: introduce napi_is_scheduled helper

We currently have napi_if_scheduled_mark_missed that can be used to
check if napi is scheduled but that does more thing than simply checking
it and return a bool. Some driver already implement custom function to
check if napi is scheduled.

Drop these custom function and introduce napi_is_scheduled that simply
check if napi is scheduled atomically.

Update any driver and code that implement a similar check and instead
use this new helper.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>