lib: add pr_debug() macro

Add the pr_debug() macro so that:

  #ifdef DEBUG
  	printf("foo");
  #endif

can be replaced by:

  	pr_debug("foo");

Apply the pr_debug() macro wherever relevant.

Currently, there is no consensus whether debug messages should be
printed on stdout or stderr. Most of the modules: canbusload.c,
candump.c and canlogserver.c use stdout but
mcp251xfd/mcp251xfd-dev-coredump.c uses stderr. Harmonize the behavior
by following the major trend and make
mcp251xfd/mcp251xfd-dev-coredump.c also output to stdout.

slcanpty.c does a #define DEBUG, meaning that debug is always turned
on for this file. Remove this and make debug an option like every
other files.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/all/20221114163848.3398-5-mailhol.vincent@wanadoo.fr
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

slcanpty.c

@@ -49,8 +49,6 @@
 #define SLC_MTU (sizeof("T1111222281122334455667788EA5F\r")+1)
 #define DEVICE_NAME_PTMX "/dev/ptmx"
 
-#define DEBUG
-
 /* read data from pty, send CAN frames to CAN socket and answer commands */
 int pty2can(int pty, int socket, struct can_filter *fi,
 	    int *is_open, int *tstamp)
@@ -106,14 +104,12 @@ rx_restart:
 	cmd = buf[0];
 	buf[nbytes] = 0;
 
-#ifdef DEBUG
 	for (tmp = 0; tmp < nbytes; tmp++)
 		if (buf[tmp] == '\r')
-			putchar('@');
+			pr_debug("@");
 		else
-			putchar(buf[tmp]);
-	printf("\n");
-#endif
+			pr_debug("%c", buf[tmp]);
+	pr_debug("\n");
 
 	/* check for filter configuration commands */
 	if (cmd == 'm' || cmd == 'M') {