comedi: das16: Use struct_size() helper in comedi_alloc_spriv()

comedi: das16: Use struct_size() helper in comedi_alloc_spriv()

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/comedi/drivers/das16.c:964:45: warning: using sizeof on a flexible structure
drivers/comedi/drivers/das16.c:998:45: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20220125232525.GA80228@embeddedor
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>