hwmon: (pwm-fan) Store tach data separately

The data for the (optional) fan tachometer input is moved to a separate
structure which is only allocated if an input is actually configured.

After this change the pulse IRQ handler takes a pointer to the
tachometer data structure instead of the whole device context.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Link: https://lore.kernel.org/r/20201212195008.6036-2-pbarker@konsulko.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>