rtlwifi: Fix smatch warnings in usb.c

Smatch displays the following:
  CHECK   drivers/net/wireless/rtlwifi/usb.c
drivers/net/wireless/rtlwifi/usb.c:458 _rtl_usb_rx_process_agg() warn: assigning (-98) to unsigned variable 'stats.noise'
drivers/net/wireless/rtlwifi/usb.c:503 _rtl_usb_rx_process_noagg() warn: assigning (-98) to unsigned variable 'stats.noise'
drivers/net/wireless/rtlwifi/usb.c:596 _rtl_rx_get_padding() info: ignoring unreachable code.
drivers/net/wireless/rtlwifi/usb.c:596 _rtl_rx_get_padding() info: ignoring unreachable code.

The variable 'stats.noise' is not used, thus the initializers are removed.
The unreachable code info is fixed by including the appropriate section inside
#ifdef .. #endif constructions.

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