mhi: device: netdev: Add flag to track napi scheduling

Flag is set to true when napi_schedule is called and false
when napi_complete is called. This helps to identify the
status of napi if a data stall is reported on downlink.

Change-Id: Id54340c0274da3120f21154c0774e035a61347a9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>