samples: ipc: icmsg: fix unbalanced nRF53 cpunet enable calls

It looks like sample called nrf53_cpunet_enable(false) before any
nrf53_cpunet_enable(true), resulting in asserts due to unbalanced calls
(error propagated from onoff service).

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>