spi: sun4i: Add rx_buf NULL pointer check

Current driver doesn't check if the destination pointer is NULL.
This cause the data from the FIFO to be stored inside the internal
SDRAM ( address 0 ).

The patch add simple check if the destination pointer is NULL.

Signed-off-by: Stefan Mavrodiev <stefan@olimex.com>
Acked-by: Jagan Teki <jagan@openedev.com>
[jagan: fix commit message]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>