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

Fix:
usb.c: In function 'usb_parse_config':
usb.c:331:17: warning: variable 'ch' set but not used [-Wunused-but-set-variable]
usb.c: In function 'usb_hub_port_connect_change':
usb.c:1123:29: warning: variable 'portchange' set but not used [-Wunused-but-set-variable]
usb.c: In function 'usb_hub_configure':
usb.c:1183:25: warning: variable 'hubsts' 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>