Expose net_driver_s tx buffer status to implement POLLOUT

[Peter van der Perk <pe...@nxp.com>]

Hi,

For SocketCAN I would like to implement the POLLOUT functionality.
However this means I've to query the driver check if the internal TX buffer is full.
My first thought would be extending struct net_driver_s with the following callback
int (*d_txfull)(FAR struct net_driver_s *dev);
Would this make sense or is there a better way to query the driver from network stack?

Kind regards
Peter van der Perk