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

include/linux/math64.h

@@ -120,6 +120,8 @@ extern s64 div64_s64(s64 dividend, s64 divisor);
  * This is the most common 64bit divide and should be used if possible,
  * as many 32bit archs can optimize this variant better than a full 64bit
  * divide.
+ *
+ * Return: dividend / divisor
  */
 #ifndef div_u64
 static inline u64 div_u64(u64 dividend, u32 divisor)
@@ -133,6 +135,8 @@ static inline u64 div_u64(u64 dividend, u32 divisor)
  * div_s64 - signed 64bit divide with 32bit divisor
  * @dividend: signed 64bit dividend
  * @divisor: signed 32bit divisor
+ *
+ * Return: dividend / divisor
  */
 #ifndef div_s64
 static inline s64 div_s64(s64 dividend, s32 divisor)