misc/atmel_tc: make atmel_tc.tcb_config member point to const data

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

drivers/misc/atmel_tclib.c: In function 'tc_probe':
drivers/misc/atmel_tclib.c:170: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Uwe Kleine-König <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>

include/linux/atmel_tc.h

···
 	struct platform_device	*pdev;
 	struct resource		*iomem;
 	void __iomem		*regs;
-	struct atmel_tcb_config	*tcb_config;
+	const struct atmel_tcb_config *tcb_config;
 	int			irq[3];
 	struct clk		*clk[3];
 	struct list_head	node;