usb: phy: bcm-kona-usb2: Use PTR_ERR_OR_ZERO

PTR_ERR_OR_ZERO simplifies the code.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Matt Porter <mporter@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>