usb: gadget: Fix support for 2 serial ports over char_bridge

Currently, u_data_bridge driver specifies that we can
have 2 port instances supported over it. But there is
a bug in the setup function to check the number of ports.
Fix the bug and also add corresponding port_num support
in f_serial driver.

Change-Id: I564914ff737bee8bd89e08153190d65219d002cd
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>