common/cmd_usb.c: fix warning: variable ... set but not used

Fix:
cmd_usb.c: In function 'usb_show_tree_graph':
cmd_usb.c:284:29: warning: variable 'port' set but not used [-Wunused-but-set-variable]

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