staging: gdm724x: cleanup alloc_tx_sdu_struct()

The kfree(t_sdu->buf) sets off a private static checker warning because
"t_sdu->buf" is always NULL.

This function just allocates two pointers so we can re-write it to be
simpler.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>