usb: otg: Remove the unneeded NULL check

The function usb_add_phy trusts the sanity of the caller.
Also it accesses x after the NULL check.
Remove the unneeded check.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>