net/mlx5: add define of LRO segment chunk size

Maximal size of coalesced LRO segment is set in TIR attributes as
number of chunks of size 256 bytes each.
Current implementation uses the hardcoded value 256 in several places.

This patch adds a definition for this value, and uses this definition
in all relevant places.
A debug message is added to clearly notify the actual configured size.

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>