net/mlx5: SF, Fix return type

Fix the following coccicheck warnings:

drivers/net/ethernet/mellanox/mlx5/core/sf/dev/dev.h:50:8-9: WARNING:
return of 0/1 in function 'mlx5_sf_dev_allocated' with return type bool

Signed-off-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>