USB Storage: Make driver less chatty when it finds a new device

Use dev_dbg() instead of an unconditional printk(KERN_DEBUG).  This has
two benefits; one is that it identifies the USB device which the messages
related to, and the other is that the messages won't be produced unless
debug is turned on.

Enable the debug messages when CONFIG_USB_STORAGE_DEBUG is set.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>