drivers: adc: adc_ad559x: fix driver init

k_thread_name_set() function returns an error if CONFIG_THREAD_NAME is
not enabled which may make the driver not functional.

Fix the issue by adding a check against CONFIG_THREAD_NAME avaliability
before calling k_thread_name_set() function so the driver can operate
properly with CONFIG_THREAD_NAME disabled.

Signed-off-by: Lukasz Madej <l.madej@grinn-global.com>