dt-bindings: phy: brcm,brcmstb-usb-phy: Fix error in "compatible" conditional schema

The conditional if/then schema has an error as the "enum" values have
"const" in them. Drop the "const".

Signed-off-by: Rob Herring <robh@kernel.org>
Fixes: 46b616c1 ("dt-bindings: phy: brcm, brcmstb-usb-phy: add BCM4908 binding")
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Message-ID: <20230621230958.3815818-1-robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>