usb: dwc3: avoid probe deferral when USB power supply is not available

The dwc3 driver currently defers probe if the USB power supply is not yet
registered. On some platforms, even though charging and power supply
functionality is available during normal operation, there may exist
minimal booting modes (such as recovery or diagnostic environments) where
the relevant USB power supply device is not registered. In such cases,
probe deferral prevents USB gadget operation entirely.

USB data functionality for basic operation does not inherently depend on
the power supply framework, which is only required for enforcing VBUS
current control. The configured VBUS current limit is typically enforced
through the charger or PMIC power path. When charging functionality is
unavailable, applying a current limit has no practical effect, reducing
the benefit of strict probe-time enforcement in these environments.

Instead of deferring probe, register a power supply notifier when the
USB power supply is not yet available. Cache the requested VBUS current
limit and apply it once the matching power supply becomes available, as
notified through the registered callback.

Signed-off-by: Elson Serrao <elson.serrao@oss.qualcomm.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://patch.msgid.link/20260608171953.1717369-1-elson.serrao@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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