hwmon: (aspeed-pwm-tacho) increase fan tach period

The previous value reduced the time required to determine
the fan value, however, it's also used as the final timeout
mechanism.  The prevous value would work for any fan speed
greater than around 3k RPM.  This increased value, lets the fan
speeds return quickly but will wait longer to handle speeds below 3k
RPM.

Testing: this value was determined through experimentation on the ast2400
on the Quanta-q71l.  This configurations runs 8 fans attached to the
controller.

Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>