math64: fix kernel-doc return value warnings

Fix the following kernel-doc warnings by adding a description for return
values of div_[us]64.

	math64.h:126: warning: No description found for return value of 'div_u64'
	math64.h:139: warning: No description found for return value of 'div_s64'

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Link: https://lore.kernel.org/r/20221118182309.3824530-3-liambeguin@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>