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

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/174
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20220120020155.GA76981@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>