xhci: refactor and cleanup endpoint initialization.

xhci_endpoint_init() and helper functions were a bit messy.
Adding the higher bandwidth SuperSpeedPlus Isoc support on
top of it would make it even harder to read.

No functional changes.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>