USB: don't choose configs with no interfaces

This patch (as1372) fixes a bug in the routine that chooses the
default configuration to install when a new USB device is detected.
The algorithm is supposed to look for a config whose first interface
is for a non-vendor-specific class.  But the way it's currently
written, it will also accept a config with no interfaces at all, which
is not very useful.  (Believe it or not, such things do exist.)

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Andrew Victor <avictor.za@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>