xhci: rework xhci extended capability list parsing functions

Replace the existing two extended capability parsing helper functions with
one called xhci_find_next_ext_cap().

The extended capabilities are read both in pci-quirks before xhci driver is
loaded, and inside the xhci driver when adding ports. The existing helpers
did not suit well for these cases and a lot of custom parsing code was
needed.

The new helper function simplifies these two cases a lot.

The motivation for this rework was that code to support xhci debug
capability needed to parse extended capabilities, and it included
yet another capability parsing helper specific for its needs. With
this solution it debug capability code can use this new  helper as well

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