net: context: Improve packet socket interface selection on sendto()

Currently, the packet socket implementation in net_context required that
netowrk interface to transmit the packet to was set on every sendto()
call. This spoils the whole idea of binding a socket. Fix this, by
checking first if the net_conext is already bound to a particular
interface, and if so, do not throw an error in case sll_ifindex field is
not a valid interface.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>