net/mlx5: FPGA, Move FPGA init/cleanup to init_once

The FPGA init and cleanup routines should be called just once per
device.
Move them to the init_once and cleanup_once routines.

Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>