usb: gadget: function: f_obex: fix Interface Descriptor Test

On USB20CV's Interface Descriptor Test, a series
of SetInterface/GetInterface requests are issued
and gadget driver is required to always return
correct alternate setting.

In one step of the test, g_serial with f_obex
was returning the wrong value (1 instead of 0).

In order to fix this, we will now hold currently
selected alternate setting inside our struct f_obex
and just return that from our ->get_alt()
implementation.

Note that his also simplifies the code a bit.

Signed-off-by: Felipe Balbi <balbi@ti.com>