USB: usb-storage: trivial debug improvements

Use pr_foo and dev_foo instead of printk. Maybe US_DEBUG* should be
replaced too.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>