mlxsw: spectrum_buffers: Inline mlxsw_sp_sb_max_headroom_cells()

This function is now only used from the buffers module, and is a trivial
field reference. Just inline it and drop the related artifacts.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>