usb: atm: 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 or integer overflows that,
in the worse scenario, could lead to heap overflows.

Link: https://github.com/KSPP/linux/issues/160


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210928233935.GA299525@embeddedor


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>