spi: Minor cleanup

spi: Minor cleanup

- Add comments on mode_rx
- Tab space's

Cc: Simon Glass <sjg@chromium.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Tested-by: Mugunthan V N <mugunthanvnm@ti.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Jagan Teki <jteki@openedev.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>