Build fixes for struct pt_regs removal

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
diff --git a/drivers/serial/mux.c b/drivers/serial/mux.c
index aa819d3..8ad1b8c 100644
--- a/drivers/serial/mux.c
+++ b/drivers/serial/mux.c
@@ -230,7 +230,7 @@
 				continue;
 		}
 
-		if (uart_handle_sysrq_char(port, data & 0xffu, NULL))
+		if (uart_handle_sysrq_char(port, data & 0xffu))
 			continue;
 
 		tty_insert_flip_char(tty, data & 0xFF, TTY_NORMAL);