USB: serial: f81534: only read configuration once

In the original code, We'll read configuration in calc_num_ports()
and read again in attach(). In fact, we can move all content from
attach() to calc_num_ports() to simplify the code.

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
[ johan: replace commit summary ]
Signed-off-by: Johan Hovold <johan@kernel.org>