rtlwifi: rtl8192cu: Fix W=1 build warning

When this driver is built with "make W=1", the following warning is output:

drivers/net/wireless/rtlwifi/rtl8192cu/sw.c:56:6: warning: variable ‘err’ 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>