kernel-doc: ignore case when stripping attributes

There are valid attributes that could have upper case letters, but we
still want to remove, like for example
	__attribute__((aligned(NETDEV_ALIGN)))
as encountered in the wireless code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>