staging: r8188eu: remove rtw_buf_free() function

It is just a wrapper around kfree(), so just replace all calls with
kfree() instead.  The pointer is also set to NULL and the length set to
0 to emulate the original logic, just to be "safe" as I do not have a
device to test with, but odds are that can be removed later on.

Link: https://lore.kernel.org/r/20210730092417.1014392-8-gregkh@linuxfoundation.org


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