samples: net: irc_bot: modify disco check in on_context_recv()

Currently, status can be 0 when a NULL buffer is sent to
on_context_recv() so let's not include status in our check.

And in the future, status parameter may be a non-zero value indicating
an error condition.  So modify check to include positive values as well.

NOTE: Also make sure to use NET_ERR in these cases and not NET_INFO.

Change-Id: I8eac9a86d22b3b5624cd7a4e9d99e57afcac035f
Signed-off-by: Michael Scott <michael.scott@linaro.org>