drivers: display: stm32_ltdc: fix return value

[JarmouniA]

Blanking On/Off calls should not return 0 when there is no panel controller to forward them to, instead they should return ENOSYS to signal to the application that they were not actually executed.

"device_is_ready" does check for null, but also for "dev->state->initialized == false", so we need to isolate the "dev == NULL" case.

Fixes #75265

[erwango]

@kartben Strange issue of the welcoming script:

Error: Validation Failed: {"message":"The listed users cannot be searched either because the users do not exist or you do not have permission to view the users.","resource":"Search","field":"q","code":"invalid"}

[kartben]

@kartben Strange issue of the welcoming script:

Error: Validation Failed: {"message":"The listed users cannot be searched either because the users do not exist or you do not have permission to view the users.","resource":"Search","field":"q","code":"invalid"}

It happens very rarely so I never really bothered to fix it ; it's because @JarmouniA's GitHub profile is set to private.

[JarmouniA]

Cc @jfischer-no @danieldegrasse :)