sfc: Convert to use ERR_CAST()

As opposed to open-code, using the ERR_CAST macro clearly indicates that
this is a pointer to an error value and a type conversion was performed.

Signed-off-by: Shen Lichuan <shenlichuan@vivo.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Martin Habets <habetsm.xilinx@gmail.com>
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/20240829021253.3066-1-shenlichuan@vivo.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sfc/tc_counters.c

@@ -249,7 +249,7 @@ struct efx_tc_counter_index *efx_tc_flower_get_counter_index(
 					       &ctr->linkage,
 					       efx_tc_counter_id_ht_params);
 			kfree(ctr);
-			return (void *)cnt; /* it's an ERR_PTR */
+			return ERR_CAST(cnt);
 		}
 		ctr->cnt = cnt;
 		refcount_set(&ctr->ref, 1);