media: cx18: Replace custom implementation of list_entry_is_head()

Besides list_entry_is_past_end() is the same as list_entry_is_head(),
it's implemented in the list namespace. Fix both of the issue by replacing
the custom version with list_entry_is_head() calls.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>