sfc: add functions to allocate/free MAE counters

efx_tc_flower_get_counter_index() will create an MAE counter mapped to
 the passed (TC filter) cookie, or increment the reference if one already
 exists for that cookie.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>