sf: sf_ops: Stop leaking memory

It's usually a common pattern to free() the memory that we allocated.
Implement this here to stop leaking memory.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>