tty: rpmsg: Assign returned id to a local variable

Instead of putting garbage in the data structure, assign allocated id
or an error code to a temporary variable. This makes code cleaner.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211025135148.53944-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>