zbus: separate clang-format control comments

Add a line of whitespace between `/* clang-format off */` comments and
the first line of doxygen comment blocks. Some intellisense parsers
will combine all sequential comment blocks when providing code context.
This results in the control comment appearing in the help text for
`ZBUS_CHAN_DEFINE`, for example.

Signed-off-by: Jordan Yates <jordan@embeint.com>