rtlwifi: Fix warnings in usb.c

Building this driver with "make W=1" results in the following 2 warnings:

drivers/net/wireless/rtlwifi/usb.c:829:21: warning: variable ‘urb_list’ set but not used [-Wunused-but-set-variable]
drivers/net/wireless/rtlwifi/usb.c:828:23: warning: variable ‘skb_list’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>