drivers: usb_dc_mcux_ehci: fix endpoints index assignment

MCUX EHCI USB device controller supports a specific
number of bidirectional endpoints. Bidirectional means
that an endpoint object is represented to the outside
as an OUT and an IN Eindpoint with its own buffers
and control structures.

ep_abs_idx index refers to the corresponding control
structure, for example:

 EP addr | ep_idx | ep_abs_idx
-------------------------------
 0x00    | 0x00   | 0x00
 0x80    | 0x00   | 0x01
 0x01    | 0x01   | 0x02
 0x81    | 0x01   | 0x03
 ....    | ....   | ....

The NUM_OF_EP_MAX (and number of s_ep_ctrl) should be double
of num_bidir_endpoints. There is also no need to reserve
endpoint addresses for this controller type.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>