usb: gadget: f_phonet: Use struct_size() helper in kzalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/usb/gadget/function/f_phonet.c:673:16: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/160
Link: https://github.com/KSPP/linux/issues/174Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20220120020155.GA76981@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_phonet.c

@@ -668,10 +668,8 @@ static struct usb_function *phonet_alloc(struct usb_function_instance *fi)
 {
 	struct f_phonet *fp;
 	struct f_phonet_opts *opts;
-	int size;
 
-	size = sizeof(*fp) + (phonet_rxq_size * sizeof(struct usb_request *));
-	fp = kzalloc(size, GFP_KERNEL);
+	fp = kzalloc(struct_size(fp, out_reqv, phonet_rxq_size), GFP_KERNEL);
 	if (!fp)
 		return ERR_PTR(-ENOMEM);