[media] media-entity.h: Correct KernelDoc of media_entity_enum_empty()

The function media_entity_enum_empty() returns true when the bitmap
of the input parameter media entity enumeration is empty instead of marked.
This patch corrects the return value description of the function.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>