ubus: Correct usage of timeout on poll function

ubus: Correct usage of timeout on poll function

As the man page explains:

"Specifying a timeout of zero causes poll() to return immediately,
even if no file descriptors are ready."

The use of 0 as timeout could cause libubus to busy loop if the
socket was non-blocking. For blocking sockets, this was less
apparent as the subsequent read() would then block.

Signed-off-by: Karl Vogel <karl.vogel@gmail.com>