x86/entry: Fix kernel-doc warning

The do_int80_emulation() function is missing a kernel-doc formatted
description of its argument. This is causing a warning when building
with W=1. Add a brief description of the argument to satisfy
kernel-doc.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241219155227.685692-1-daniel.sneddon@linux.intel.com
Closes: https://lore.kernel.org/oe-kbuild-all/202412131236.a5HhOqXo-lkp@intel.com/