net: sockets: Stick to NET_SOCKETS_POLL_MAX limit in socket services

Socket service pollfd count should not exceed the configured
NET_SOCKETS_POLL_MAX limit, as poll() will not be able to monitor
sockets beyond that limit anyway. Adding +1 there prevented the library
from catching the configuration error.

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