staging: rtl8192u: remove unused static variable

remove unused static variable channels.

Signed-off-by: Saurav Girepunje <saurav.girepunje@gmail.com>
Link: https://lore.kernel.org/r/YVjN9FT4KjLGX/Zg@user


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>