hwmon: (aspeed-pwm-tacho) Poll with short sleeps.

The reference driver polled but mentioned it was possible to sleep
for a computed period to know when it's ready to read.  However, polling
with minimal sleeps is quick and works.  This also improves responsiveness
from the driver.

Testing: tested on ast2400 on quanta-q71l

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