phy: Return success from generic_setup_phy() when phy is not found

Restore the old behavior of ehci_setup_phy() and ohci_setup_phy() to
return success when generic_phy_get_by_index() return -ENOENT.

Fixes: 84e56140 ("phy: Add generic_{setup,shutdown}_phy() helpers")
Fixes: 10005004 ("usb: ohci: Make usage of generic_{setup,shutdown}_phy() helpers")
Fixes: 083f8aa9 ("usb: ehci: Make usage of generic_{setup,shutdown}_phy() helpers")
Fixes: 75341e9c ("usb: ehci: Remove unused ehci_{setup,shutdown}_phy() helpers")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>