hwmon: (aspeed-pwm-tacho) On read failure return -ETIMEDOUT

When the controller fails to provide an RPM reading within the alloted
time; the driver returns -ETIMEDOUT and no file contents.

Signed-off-by: Patrick Venture <venture@google.com>
Fixes: 2d7a548a ("drivers: hwmon: Support for ASPEED PWM/Fan tach")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>