xpp: revert USB "clear_halt" change and better overrides.

Why:
 * Doing "clear_halt" is normally the right thing to do on startup.
 * The original observed problem is better fixed via USB bios settings.
 * Defaulting to no "clear_halt" cause more problems on other platform
   combinations (hardware/kernel).

The change:
 * We now reverted to do "clear_halt" by default.
 * The XTALK_OPTIONS may now contain either "use-clear-halt" (the default)
   or "no-use-clear-halt" to override for debugging/testing.

Original commit: ca7c04e9cbdfc182c9c9672dec884f5ac87f90b9

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>