staging: rtl8188eu: refactor rtw_is_cckrates_included()

Refactor rtw_is_cckrates_included() to improve readability and
slightly reduce object file size.

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