staging: pi433: reduce stack size in tx thread

Putting a 900 byte array on the stack is a clearly too much in the
kernel, and sometimes results in warnings like:

drivers/staging/pi433/pi433_if.c: In function 'pi433_tx_thread':
drivers/staging/pi433/pi433_if.c:645:1: error: the frame size of 1028 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

This moves the buffer into the dynamically allocated per-device
structure.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Marcus Wolf <linux@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

@@ -92,6 +92,7 @@ struct pi433_device {
 	struct task_struct	*tx_task_struct;
 	wait_queue_head_t	tx_wait_queue;
 	u8			free_in_fifo;
+	char			buffer[MAX_MSG_SIZE];
 
 	/* rx related values */
 	struct pi433_rx_cfg	rx_cfg;
@@ -490,7 +491,7 @@ pi433_tx_thread(void *data)
 	struct pi433_device *device = data;
 	struct spi_device *spi = device->spi; /* needed for SET_CHECKED */
 	struct pi433_tx_cfg tx_cfg;
-	u8     buffer[MAX_MSG_SIZE];
+	u8     *buffer = device->buffer;
 	size_t size;
 	bool   rx_interrupted = false;
 	int    position, repetitions;