staging: rtl8188eu: Remove code valid only for 5 GHz

Remove code that is only valid for channels > 14.
This addresses the below TODO item.

- find and remove remaining code valid only for 5 GHz. Most of the obvious
  ones have been removed, but things like channel > 14 still exist.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>