hwmon: (emc2305) Fix fan channel index handling

hwmon: (emc2305) Fix fan channel index handling

The fan channel index is used to access per-channel data structures.
Validate the index against the number of available channels
before use to prevent out-of-bounds access if an invalid
value is provided.

The thermal registration path currently uses a sequential child index,
which may not match the validated channel from DT. Use the DT "reg"
property when registering cooling devices to ensure consistent
channel handling

Signed-off-by: Florin Leotescu <florin.leotescu@nxp.com>
Link: https://lore.kernel.org/r/20260429065955.2113012-2-florin.leotescu@oss.nxp.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>