usb: ehci-hcd: Fix crash when no ops are provided to ehci_register()

This commit fixes crash on BananaPi (and possibly others)
casued by 3f9f8a5b.

Crash reason:
When no ops were passed to ehci_register(), USB host driver caused
NULL pointer dereference.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>