usb: host: ehci-sched: Use struct_size() in kzalloc()

usb: host: ehci-sched: Use struct_size() 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 worse scenario, could lead to heap overflows.

Also, address the following sparse warning:
drivers/usb/host/ehci-sched.c:1168:40: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/160
Link: https://github.com/KSPP/linux/issues/174
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20220111075427.GA76390@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>