thermal: core: Do not fail cdev registration because of invalid initial state

It is reported that commit 31a0fa00 ("thermal/debugfs: Pass cooling
device state to thermal_debug_cdev_add()") causes the ACPI fan driver
to fail probing on some systems which turns out to be due to the _FST
control method returning an invalid value until _FSL is first evaluated
for the given fan.  If this happens, the .get_cur_state() cooling device
callback returns an error and __thermal_cooling_device_register() fails
as uses that callback after commit 31a0fa00.

Arguably, _FST should not return an invalid value even if it is
evaluated before _FSL, so this may be regarded as a platform firmware
issue, but at the same time it is not a good enough reason for failing
the cooling device registration where the initial cooling device state
is only needed to initialize a thermal debug facility.

Accordingly, modify __thermal_cooling_device_register() to avoid
calling thermal_debug_cdev_add() instead of returning an error if the
initial .get_cur_state() callback invocation fails.

Fixes: 31a0fa00

 ("thermal/debugfs: Pass cooling device state to thermal_debug_cdev_add()")
Closes: https://lore.kernel.org/linux-acpi/20240530153727.843378-1-laura.nao@collabora.com
Reported-by: Laura Nao <laura.nao@collabora.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Laura Nao <laura.nao@collabora.com>