mlx4: avoid init errors when kernel modules are not loaded

mlx4: avoid init errors when kernel modules are not loaded

Mimic UIO/VFIO drivers behavior by not causing errors when a device cannot
be initialized due to missing or mismatching kernel modules. Display helpful
messages instead, such as:

 [...]
 EAL: PCI device 0000:83:00.0 on NUMA socket 1
 EAL:   probe driver: 15b3:1007 librte_pmd_mlx4
 PMD: librte_pmd_mlx4: PCI information matches, using device "mlx4_0" (VF: false)
 PMD: librte_pmd_mlx4: cannot use device, are drivers up to date?
 EAL: PCI device 0000:84:00.0 on NUMA socket 1
 EAL:   probe driver: 15b3:1007 librte_pmd_mlx4
 PMD: librte_pmd_mlx4: PCI information matches, using device "mlx4_1" (VF: false)
 PMD: librte_pmd_mlx4: cannot use device, are drivers up to date?
 EAL: No probed ethernet devices
 [...]

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>