net/mlx5e: Support the flash device ethtool callback

This callback further invokes the mlxfw module to flash the new
firmware file to the device.

As the firmware flash process takes about 20 seconds and ethtool
takes the rtnl lock during the flash_device callback, we release
the rtnl lock at the beginning of the flash process and take it
again before leaving the callback.

This way, rtnl is not held during the process. To make sure the
device does not get deleted while being flashed, we take a
reference to it before releasing rtnl lock.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>