staging: rtl8188eu: remove unused rtw_get_oper_choffset()

The function rtw_get_oper_choffset() is never used, so remove it.
Discovered by cppcheck.

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