meta-romulus: phosphor-hwmon workaround for fan speed

Romulus uses aspeed pwm and fan tacho, and its driver returns ETIMEDOUT
for fan speed when host is off, which causes phosphor-hwmon exit with
excpetion for fan_tacho sensors.

The workaround is to handle ETIMEDOUT and make it return -ETIMEDOUT
value for the sensor.

Change-Id: I1c6252c7f0ba1c932948397a9398ff80dfb71e9d
Signed-off-by: Lei YU <mine260309@gmail.com>