usb: dwc2: Avoid delay when initializing USB peripheral by dwc2

When `usb start` is called on the terminal, the dwc2 driver will try
to start every USB device as host first, even if it is explicitly
configured as peripheral in the device tree (dr_mode = "peripheral").

So to avoid an unwanted 15 seconds delay when initializing the usb
(one second per channel = 1s x 15), this patch adds a check to the
initialization, and will skip host initialization of the device is
explicitly set as peripheral. The checking is already done similarly
in the `drivers/usb/gadget/dwc2_udc_otg.c` driver.

Signed-off-by: João Loureiro <joaofl@gmail.com>