mISDN: hfcsusb: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/hardware/mISDN/hfcsusb.c

@@ -261,8 +261,7 @@ hfcsusb_ph_info(struct hfcsusb *hw)
 		phi->bch[i].Flags = hw->bch[i].Flags;
 	}
 	_queue_data(&dch->dev.D, MPH_INFORMATION_IND, MISDN_ID_ANY,
-		    sizeof(struct ph_info_dch) + dch->dev.nrbchan *
-		    sizeof(struct ph_info_ch), phi, GFP_ATOMIC);
+		    struct_size(phi, bch, dch->dev.nrbchan), phi, GFP_ATOMIC);
 	kfree(phi);
 }