net/mlx5: ft: Use getter function to get ft chain

FT chain is defined as the next chain after tc.

To prepare for next patches that will increase the number of tc
chains available at runtime, use a getter function to get this
value.

The define is still used in static fs_core allocation,
to calculate the number of chains. This static allocation
will be used if the relevant capabilities won't be available
to support dynamic chains.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>