serial: 8250_pci: Prefer struct_size over open coded arithmetic

As noted in the "Deprecated Interfaces, Language Features, Attributes,
and Conventions" documentation [1], size calculations (especially
multiplication) should not be performed in memory allocator (or similar)
function arguments due to the risk of them overflowing. This could lead
to values wrapping around and a smaller allocation being made than the
caller was expecting. Using those allocations could lead to linear
overflows of heap memory and other misbehaviors.

So, use the struct_size() helper to do the arithmetic instead of the
argument "size + size * count" in the kzalloc() function.

[1] https://www.kernel.org/doc/html/v5.14/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Len Baker <len.baker@gmx.com>
Link: https://lore.kernel.org/r/20210905155728.11147-1-len.baker@gmx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>