usb: ehci-mx6: allow board_ehci_hcd_init to fail

There could be runtime determined board specific reason why a EHCI
initialization fails (e.g. ENODEV if a Port is not available). In
this case, properly return the error code.
While at it, that function (board_ehci_hcd_init) has actually two
documentation blocks... Use the correct function name for the
documentation block of board_usb_phy_mode.

Signed-off-by: Stefan Agner <stefan@agner.ch>