USB: storage: avoid scanning other targets for single target device

This patch sets scsi_host->max_id as 1 if the device's quirk
flag of US_FL_SCM_MULT_TARG isn't set, because there are only 6
mass storage devices marked as mutiple targets from unusual_devs.h.

This patch is a small optimization about scanning targets, and
avoid scanning other 7 non-existed targets for single target
device.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>