usb: dwc2: Make PHY optional

Fixes commit 09a75e85
("usb: dwc2: refactor common low-level hw code to platform.c")

The above commit consolidated the low-level phy access into a common
location. This change introduced a check from the gadget requiring
that a PHY is specified. This requirement never existed on the host
side and broke some platforms when it was moved into platform.c.

The gadget doesn't require the PHY either so remove the check.

Acked-by: Eric Anholt <eric@anholt.net>
Reported-by: Stefan Wahren <info@lategoodbye.de>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: 09a75e85 ("usb: dwc2: refactor common low-level hw code to platform.c")
Signed-off-by: Felipe Balbi <balbi@ti.com>