doc: fix usage of :c:type

Fixing issues with recent versions of breathe 4.19.2:

 WARNING: Unparseable C cross-reference: 'struct device'
 Invalid C declaration: Expected identifier in nested name, got keyword:
 struct [error at 6]
   struct device
     ------^

Signed-off-by: Anas Nashif <anas.nashif@intel.com>