net: if: Don't check NET_IF_UP in net_if_prepare_events

NET_IF_UP may change during the lifetime of k_pool which means we would
have to reconfigure everytime the flag changes but NET_IF_UP is already
checked during net_if_send_data thus it should never reach the queue in
the first place making this check unnecessary.

Jira: ZEP-1888

Change-Id: Iaa8471bee886a6f7e701a1dd243fb199def26589
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>