rtl8xxxu: fix unused rtl8192cu_fops compiler warning

kbuild reported:

rtl8xxxu.c:5786:32: warning: ‘rtl8192cu_fops’ defined but not used [-Wunused-variable]

Fix it by adding temporary ifdefs around the static functions.

Fixes: 033695bd ("rtl8xxxu: move devices supported by rtlwifi under UNTESTED config")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>