usb: storage: Show number of storage devices detected for DM_USB

By enabling DM_USB information about number of storage devices
was lost.
Get this information back simply by printing number of devices detected
via BLK uclass.

For example:
scanning bus 0 for devices... 7 USB Device(s) found
       scanning usb for storage devices... 3 Storage Device(s) found
       scanning usb for ethernet devices... 0 Ethernet Device(s) found

Signed-off-by: Michal Simek <michal.simek@xilinx.com>