hwmon: (aspeed-pwm-tacho) reduce fan_tach period

Reduce the fan_tach period such that the fan controller uses a shorter
period to measure the rpm.

The original period of 0x1000 was chosen as a conversative value from the
reference implementation.  Through experimentation on the quanta-q71l
board, I was able to drive the number down which ultimately reduced the
time the controller would use to determine the fan_tach.  This value was
recently tested and accepted downstream on the IBM Zaius board which uses
the ast2500.

Future work: It may be worthwhile as this is a tunable parameter to the
system, to allow overriding it through the device tree.

Testing: Tested on an ast2400 sitting on a quanta-q71l and ast2500 on
power9.
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/aspeed-pwm-tacho.c

@@ -160,7 +160,7 @@
  * 11: reserved.
  */
 #define M_TACH_MODE 0x02 /* 10b */
-#define M_TACH_UNIT 0x1000
+#define M_TACH_UNIT 0x00c0
 #define INIT_FAN_CTRL 0xFF
 
 struct aspeed_pwm_tacho_data {