net: sockets: fix POLLOUT for offloaded iface

For offloaded iface net_tcp_get is never called, so context->tcp
is always NULL. In that case net_tcp_tx_sem_get will return wrong pointer.
For pollout k_poll will be called with NULL semph,
which cause HardFault.

Signed-off-by: Wojciech Slenska <wsl@trackunit.com>