net/mlx5: Move flow counters data structures from flow steering header

After the following flow counters API refactoring:
("net/mlx5: Use flow counter IDs and not the wrapping cache object")
flow counters private data structures mlx5_fc_cache and mlx5_fc are
redundantly exposed in fs_core.h, they have nothing to do with flow
steering core and they are private to fs_counter.c, this patch moves them
to where they belong and reduces their exposure in the driver.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>