usb: udc: Try to clarify an error message

At some point when trying to use USB gadgets, two situations may arise
and lead to a failure. Either the UDC (USB Device Controller) is not
available at all (not described or not probed) or the UDC is already in
use. For instance, as the USB Ethernet gadget remains bound to the UDC,
the use of any other USB gadget (fastboot, dfu, etc) *after* will always
fail with the "couldn't find an available UDC" error.

Let's give a more helpful message by making a difference between the two
cases. Let's also hint people who would get this error and grep it into
the sources a better explanation of what's wrong with their workflow.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20231010090304.49335-4-miquel.raynal@bootlin.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

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