Implement set/get functions to configure PFC stall prevention
timeout by tunables api through ethtool.
By default the stall prevention timeout is configured to 8 sec.
Timeout range is: 80-8000 msec.

Enabling stall prevention with the auto timeout will set
the timeout to 100 msec.
Signed-off-by: NInbar Karmy <inbark@mellanox.com>
Signed-off-by: NSaeed Mahameed <saeedm@mellanox.com>