leds: leds-hp6xx: Use devm_led_classdev_register

Use resource-managed function devm_led_classdev_register
instead of led_classdev_register to simplify error path.
An unnecessary check of ret is replaced with a direct return
Also, remove redundant hp6xxled_remove.

Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>