hwmon: (axi-fan-control) Fix possible NULL pointer dereference

hwmon: (axi-fan-control) Fix possible NULL pointer dereference

axi_fan_control_irq_handler(), dependent on the private
axi_fan_control_data structure, might be called before the hwmon
device is registered. That will cause an "Unable to handle kernel
NULL pointer dereference" error.

Fixes: 8412b410fa5e ("hwmon: Support ADI Fan Control IP")
Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231025132100.649499-1-nuno.sa@analog.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>