hwmon: (tmp421) Allow reading at 2Hz instead of 0.5Hz

Our driver configures the device to read at 2Hz, but then only allows the
user to read cached temp values at up to 0.5Hz. Let's allow users to read
as quickly as we do.

Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Link: https://lore.kernel.org/r/20191014140310.7438-1-kyle.roeschley@ni.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>