rtlwifi: Fix smatch warning in pci.c

Smatch reports the following:
  CHECK   drivers/net/wireless/rtlwifi/pci.c
drivers/net/wireless/rtlwifi/pci.c:739 _rtl_pci_rx_interrupt() warn: assigning (-98) to unsigned variable 'stats.noise'

The variable 'stats.noise' is not used. That initializer is removed.

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