Fix poll rate on touchscreen (#3238)

Was running at 25Hz, rather than he expected 60. Only been doing it
for the last 5 years....

Replace msleep_interruptible with usleep_range as the msleep call
is not accurate for times < 20ms.

Fixes: #3227

Signed-off-by: James Hughes <james.hughes@raspberrypi.org>

drivers/input/touchscreen/rpi-ft5406.c

@@ -78,7 +78,7 @@ static int ft5406_thread(void *arg)
 
  while (!kthread_should_stop()) {
  /* 60fps polling */
- msleep_interruptible(17);
+ usleep_range(16600, 16700);
  memcpy_fromio(&regs, ts->ts_base, sizeof(struct ft5406_regs));
  iowrite8(99,
  ts->ts_base +