scripts/kernel-doc: Fix struct and struct field attribute processing

The kernel-doc attempts to clear the struct and struct member attributes
from the API documentation it produces. It falls short of the job in the
following respects:

- extra whitespaces are left where __attribute__((...)) was removed,

- only a single attribute is removed per struct,

- attributes (such as aligned) containing numbers were not removed,

- attributes are only cleared from struct fields, not structs themselves.

This patch addresses these issues by removing the attributes.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>