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: https://github.com/raspberrypi/linux/issues/3227



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