server: fix signedness in wl_client_connection_data

The variables opcode and size were unsigned, which lead to warnings
about comparisons of signed vs. unsigned.

Change these variable to signed. Their usage never relies on being
unsigned.

This also fixes (an assumed) printf format string problem, where these
were printed with %d, not %u.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>