GCC4.6: Squash warnings in common/usb.c

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: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Mike Frysinger <vapier@gentoo.org>