hostap: fixup strlen() math

In hostap_add_interface() we do:
	sprintf(dev->name, "%s%s", prefix, name);

dev->name has IFNAMSIZ (16) characters.
prefix is local->dev->name.
name is "wds%d"

strlen() returns the number of characters in the string not counting the
NULL so if we have a string with 11 characters we get "12345678901wds%d"
which is 16 characters and a NULL so we're past the end of the array.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>