ENGR00258357-5: mlb: Use circle buf macros to replace old ringbuf mechanism

ENGR00258357-5: mlb: Use circle buf macros to replace old ringbuf mechanism

Use circle buf to replace old ringbuf mechanism.
Change to use circle buffer in read, write, rx isr and tx isr functions.
In first design of MLB, it's using it's own mechanism to manage ring
buffer, like in mxc_mlb.c.
And then, I saw that kernel already had a serials of circ buffer macros
which can be used to manage ring buffers.
This patch is to use circle buffer macros to manage mlb internal ring
buffers.
For detail of circle buffers, you can refer to
linux-2.6-imx/Documentation/circular-buffers.txt.

Signed-off-by: Terry Lv <r65388@freescale.com>