Staging: pi433: declare functions static

Declare functions static to fix sparse warnings:

warning: symbol 'pi433_receive' was not declared. Should it be static?
warning: symbol 'pi433_tx_thread' was not declared. Should it be static?
Signed-off-by: Joseph Wright <rjosephwright@gmail.com>
Reviewed-by: Marcus Wolf <linux@wolf-entwicklungen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

@@ -313,7 +313,7 @@ pi433_start_rx(struct pi433_device *dev)
 
 /*-------------------------------------------------------------------------*/
 
-int
+static int
 pi433_receive(void *data)
 {
 	struct pi433_device *dev = data;
@@ -463,7 +463,7 @@ pi433_receive(void *data)
 		return bytes_total;
 }
 
-int
+static int
 pi433_tx_thread(void *data)
 {
 	struct pi433_device *device = data;