mfd: ucb1x00-assabet: Use DEVICE_ATTR_RO macro

Use DEVICE_ATTR_RO macro helper instead of plain DEVICE_ATTR, which makes
the code a bit shorter and easier to read.

Signed-off-by: Zhen Lei <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/ucb1x00-assabet.c

···
 	ucb1x00_adc_disable(ucb);				\
 	return sprintf(buf, "%d\n", val);			\
 }								\
-static DEVICE_ATTR(name,0444,name##_show,NULL)
+static DEVICE_ATTR_RO(name)
 
 UCB1X00_ATTR(vbatt, UCB_ADC_INP_AD1);
 UCB1X00_ATTR(vcharger, UCB_ADC_INP_AD0);