usb: f_serial: Remove unnecessary return statements in gport_setup

Driver allocates and initializes port associated with transport
for a serial interface by calling gport_setup. In case of HSIC
transport if port setup is successful gport_setup returns right
away without setting up hsuart port if it is present. As a result
if a usb composition has serial interface for both hsic and hsuart
transports then hsuart port will not be allocated and initialized.
This causes NULL pointer dereference of hsuart port later during
device enumeration.

Change-Id: I07959d88daace9e1a9351c19ee2d04900638a48b
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>