rtlwifi: rtl8192cu: Fix smatch warning in rtl8192cu/trx.c

Smatch lists the following:
  CHECK   drivers/net/wireless/rtlwifi/rtl8192cu/trx.c
drivers/net/wireless/rtlwifi/rtl8192cu/trx.c:367 _rtl_rx_process() warn: assigning (-98) to unsigned variable 'stats.noise'

This variable is unused, thus the initializer is removed, as well as a
comment referring to that variable.

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