platform/x86: asus-wmi: Fix support for showing middle fan RPM

After the addition of the mid fan custom curve functionality various
incorrect behaviour was uncovered. This commit fixes these areas.

- Ensure mid fan attributes actually use the correct fan ID
- Correction to a bit mask for selecting the correct fan data
- Refactor the curve show/store functions to be cleaner and
  match each others layout

Fixes: 536fce82 ("platform/x86: asus-wmi: add support for showing middle fan RPM")
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20230815014209.44903-1-luke@ljones.dev


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>