The DHT11 datasheet is pretty cryptic, but it does suggest that after
each integer value (humidity and temperature) there are "decimal"
values. Validate these as integers in the range 0-9 and treat them as
tenths of a unit.
Link: https://github.com/raspberrypi/linux/issues/6220
Signed-off-by: Phil Elwell <phil@raspberrypi.com>