net: rtl8192cu: Fix outstanding GCC 6.4.0 warnings

net: rtl8192cu: Fix outstanding GCC 6.4.0 warnings

Compiler used: toolchains.bootlin.com

Reference: https://github.com/diederikdehaas/rtl8812AU

[@nathanchance: Cherry-picked from 6c1987881822 and updated message]
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
(cherry picked from commit 4628e36c0caaa7806ffcd98286eab2705d37e3be
https://github.com/raspberrypi/linux raspi/rpi-4.15.y)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>