mgmt: hawkbit: optimize poll_sleep

remove unnecessary condition for CONFIG_HAWKBIT_POLL_INTERVAL,
changes poll_sleep to be in seconds,
change type of sleep_time in hawkbit_update_sleep(), so the return
value of hawkbit_time2sec() is interpreted correctly.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>