phy: bcm-ns-usb2: checking the wrong variable

We intended to test "usb2->phy" here instead of "dev".

Fixes: d3feb406

 ('phy: bcm-ns-usb2: new driver for USB 2.0 PHY on Northstar')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>