yate: increase max acceptable size of incoming SIP messages

yate: increase max acceptable size of incoming SIP messages

Some SIP UAs support lots of features and codecs which results in
large SIP messages. YATE, with its default configuration, truncates
and fails to parse received SIP messages which are larger than 1500
bytes. Let's increase the default max message size for OpenWrt users
to make it easier to use yate out-of-the-box.

The new max size of 8192 bytes has been arbitrarily chosen.

I've seen the SIP UA baresip produce messages larger than 1500 bytes
with its default configuration when authentication is used.

Signed-off-by: Robert Högberg <robert.hogberg@gmail.com>