HID: hyperv: avoid struct memcpy overrun warning

A previous patch addressed the fortified memcpy warning for most
builds, but I still see this one with gcc-9:

In file included from include/linux/string.h:254,
                 from drivers/hid/hid-hyperv.c:8:
In function 'fortify_memcpy_chk',
    inlined from 'mousevsc_on_receive' at drivers/hid/hid-hyperv.c:272:3:
include/linux/fortify-string.h:583:4: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  583 |    __write_overflow_field(p_size_field, size);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My guess is that the WARN_ON() itself is what confuses gcc, so it no
longer sees that there is a correct range check. Rework the code in a
way that helps readability and avoids the warning.

Fixes: 542f25a9

 ("HID: hyperv: Replace one-element array with flexible-array member")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20230705140242.844167-1-arnd@kernel.org
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>