omap2/3/4: serial: remove initialization sparse warnings

Initialization of pointer should be done with NULL. Removes sparse
warnings:
arch/arm/mach-omap2/serial.c:566:17: warning: Using plain integer as NULL pointer
arch/arm/mach-omap2/serial.c:567:17: warning: Using plain integer as NULL pointer

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/serial.c

@@ -495,8 +495,8 @@ static void omap_uart_idle_init(struct omap_uart_state *uart)
 		}
 		uart->wk_mask = wk_mask;
 	} else {
-		uart->wk_en = 0;
-		uart->wk_st = 0;
+		uart->wk_en = NULL;
+		uart->wk_st = NULL;
 		uart->wk_mask = 0;
 		uart->padconf = 0;
 	}