usb_storage.c: change progress output in debug() message

The dots printed by common/usb_storage.c as progress meter corrupt the
output for example of "fatls usb" commands like this:

=> fatls usb 0
.			<<==== here
       29   file.001
       29   file.002
       29   file.003
       29   file.004
       29   file.005
       29   file.006
       29   file.007
       29   file.008
       29   file.009
       29   file.010
       29   file.011
       29   file.012
       29   file.013
       29   file.014
       29   file.015
       29   file.016
.			<<==== here
       29   file.017
       29   file.018
       29   file.019
...

Turn the progress output into a debug message.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Remy Bohmer <linux@bohmer.net>