mfd: ab8500-gpadc: Change to usleep_range() for greater resolution

The resolution of msleep is related to HZ, so with HZ set to
100 any msleep of less than 10ms will become ~10ms.

Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/ab8500-gpadc.c

@@ -344,7 +344,7 @@ int ab8500_gpadc_read_raw(struct ab8500_gpadc *gpadc, u8 channel)
 		  * Delay might be needed for ABB8500 cut 3.0, if not, remove
 		  * when hardware will be available
 		  */
-			msleep(1);
+			usleep_range(1000, 1000);
 			break;
 		}
 		/* Intentional fallthrough */