IB/mthca: Convert to use be16_add_cpu()

replace:

	big_endian_variable = cpu_to_beX(beX_to_cpu(big_endian_variable) +
						expression_in_cpu_byteorder);

with:

	beX_add_cpu(&big_endian_variable, expression_in_cpu_byteorder);

Generated with a semantic patch.
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: Hal Rosenstock <hal.rosenstock@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mthca/mthca_cq.c

@@ -473,7 +473,7 @@ static void handle_error_cqe(struct mthca_dev *dev, struct mthca_cq *cq,
 	if (!(new_wqe & cpu_to_be32(0x3f)) || (!cqe->db_cnt && dbd))
 		return;
 
-	cqe->db_cnt   = cpu_to_be16(be16_to_cpu(cqe->db_cnt) - dbd);
+	be16_add_cpu(&cqe->db_cnt, -dbd);
 	cqe->wqe      = new_wqe;
 	cqe->syndrome = SYNDROME_WR_FLUSH_ERR;