spi: Minor cleanup

spi: Minor cleanup

- Add comments on mode_rx
- Tab space's

Cc: Simon Glass <[email protected]>
Cc: Michal Simek <[email protected]>
Cc: Siva Durga Prasad Paladugu <[email protected]>
Tested-by: Mugunthan V N <[email protected]>
Tested-by: Bin Meng <[email protected]>
Tested-by: Jagan Teki <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Signed-off-by: Jagan Teki <[email protected]>