Revert "staging: r8188eu: remove rtw_buf_free() function"

This reverts commit c7e88ecb as it's
broken because somehow I forgot how pointers work...

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>