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>
diff --git a/drivers/mtd/spi/sf_ops.c b/drivers/mtd/spi/sf_ops.c
index ef91b92..85cf22d 100644
--- a/drivers/mtd/spi/sf_ops.c
+++ b/drivers/mtd/spi/sf_ops.c
@@ -421,6 +421,7 @@
 		data += read_len;
 	}
 
+	free(cmd);
 	return ret;
 }