usb: dwc3: core: remove unnecessary dev_warn()

When a SoC supports both PHY interfaces but
doesn't define HSPHY in DT/pdata, we will get
an unnecessary dev_warn() which can mislead users
into thinking that they're missing something.

Instead, let's just silently rely on a correct
default. If the HW default is wrong, then HSPHY
is required and USB won't work, this will be
motivation enough for engineers to patch their
way into a working setup.

Reported-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>