async: rename and redefine async_func_ptr

A function type is typically defined as
typedef ret_type (*func)(args..)

but async_func_ptr is not.  Redefine it.

Also rename async_func_ptr to async_func_t for _func_t suffix is more generic.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>