usbcore: add configuration_string to attribute group

This patch (as737b) does a very small cleanup of core/sysfs.c by adding
the configuration_string attribute file to the existing attribute group
instead of treating it separately.  It doesn't need this separate
treatment because unlike the other device string attributes, it changes
along with the active configuration.

The patch also fixes a simple typo (which, oddly enough, doesn't seem to
bother the compiler).

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>