mlx5: support HW packet padding

Environment variable MLX5_PMD_ENABLE_PADDING enables HW packet padding
in PCI bus transactions.

When packet size is cache aligned and CRC stripping is enabled, 4 fewer
bytes are written to the PCI bus. Enabling padding makes such packets
aligned again.

In cases where PCI bandwidth is the bottleneck, padding can improve
performance by 10%.

This is disabled by default since this can also decrease performance for
unaligned packet sizes.

Signed-off-by: Olga Shern <olgas@mellanox.com>
fix packet padding macro check

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