net/tcp: Use highest priority for TCP internal work queue

Reason why the prority was at its lowest is unknown, but now that it may
be used to send local packets (which used to be sent right away),
it seems to affect TCP scheduling in loopback mode. Raising the prority
so it matches how it was previously (i.e. sent right away) should fix
things. (Note however that this issue was not broadly present, only
sockets.tls test seemed to be affected.)

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>