drivers: usb_dc_nrfx: Isochronous endpoint cannot be bulk

Prevent use of isochronous endpoints as bulk and/or interrupt endpoint.
The issue was observed when trying to use 4 CDC ACM instances where the
4th instance would claim the isochronous IN endpoint 0x88 to be bulk.
Because the isochronous endpoints cannot respond with handshake packet
the iso endpoints cannot be used as bulk or interrupt substitue.

Properly fail endpoint check and therefore make 4 CDC ACM instances not
enumerate at all because the hardware has endpoints that only allows up
to 3 CDC ACM instances.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>