usb: dwc3-generic: set "mode" based on caller of dwc3_generic_probe()

usb: dwc3-generic: set "mode" based on caller of dwc3_generic_probe()

There are only two callers of "dwc3_generic_probe()", namely:
1. dwc3_generic_peripheral_probe()
2. dwc3_generic_host_probe()
Currently, the "mode" is set based on the device-tree node of the
platform device. Also, the DWC3 core doesn't support updating the "mode"
dynamically at runtime if it is set to "OTG", i.e. "OTG" is treated as a
separate mode in itself, rather than being treated as a mode which should
eventually lead to "host"/"peripheral".

Given that the callers of "dwc3_generic_probe()" clarify the expected
"mode" of the USB Controller, use that "mode" instead of the one
specified in the device-tree. This shall allow the USB Controller to
function both as a "Host" and as a "Peripheral" when the "mode" is "otg"
in the device-tree, based on the caller of "dwc3_generic_probe()".

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Roger Quadros <rogerq@kernel.org>