mfd: atmel-hlcdc: Clean invalid sys_clk pointer

In case the Device Tree provided the LVDS PLL instead of the GCK, the
sys_clk pointer here was assigned a PTR_ERR. From then, further code in
drivers/pwm/pwm-atmel-hlcdc.c would simply test sys_clk against NULL and
treat sys_clk as valid, causing a Oops:

Unable to handle kernel paging request at virtual address fffffffe
[fffffffe] *pgd=6ffde861, *pte=00000000, *ppte=00000000
Internal error: Oops: 37 [#1] ARM

Fixes: 3887775 ("mfd: atmel-hlcdc: Obtain essential clock for LCD operation")
Signed-off-by: Aubin Constans <aubin.constans@microchip.com>

drivers/mfd/atmel-hlcdc.c

@@ -118,6 +118,7 @@ static int atmel_hlcdc_probe(struct platform_device *pdev)
 	hlcdc->sys_clk = devm_clk_get(dev, "sys_clk");
 	if (IS_ERR(hlcdc->sys_clk)) {
 		dev_dbg(dev, "failed to get sys_clk\n");
+               hlcdc->sys_clk = NULL;
 		hlcdc->lvds_pll_clk = devm_clk_get(dev, "lvds_pll_clk");
 		if (IS_ERR(hlcdc->lvds_pll_clk)) {
 			dev_err(dev, "failed to get GCK and LVDS_PLL\n");