tty/serial: kgdb_nmi: use true,false for bool variable

Fixes coccicheck warning:

drivers/tty/serial/kgdb_nmi.c:121:6-13: WARNING: Assignment of 0/1 to bool variable
drivers/tty/serial/kgdb_nmi.c:133:2-9: WARNING: Assignment of 0/1 to bool variable
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Link: https://lore.kernel.org/r/1578881777-65475-3-git-send-email-zhengbin13@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/kgdb_nmi.c

@@ -118,7 +118,7 @@ static int kgdb_nmi_poll_one_knock(void)
 	int c = -1;
 	const char *magic = kgdb_nmi_magic;
 	size_t m = strlen(magic);
-	bool printch = 0;
+	bool printch = false;
 
 	c = dbg_io_ops->read_char();
 	if (c == NO_POLL_CHAR)
@@ -130,7 +130,7 @@ static int kgdb_nmi_poll_one_knock(void)
 		n = (n + 1) % m;
 		if (!n)
 			return 1;
-		printch = 1;
+		printch = true;
 	} else {
 		n = 0;
 	}