usb-storage: remove single-use define for debugging

USB_STORAGE was defined as "usb-storage: " and used in a single location
as argument to printk. In order to be able to use the name
'USB_STORAGE', drop the definition and use the string directly for the
printk call.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>