xhci: Remove some unnecessary casts and tidy some endian swap code

Some of the recently-added cpu_to_leXX and leXX_to_cpu made things somewhat
messy; this patch neatens some of these areas, removing unnecessary casts
in those parts also.  In some places (where Y & Z are constants) a
comparison of (leXX_to_cpu(X) & Y) == Z has been replaced with
(X & cpu_to_leXX(Y)) == cpu_to_leXX(Z).  The endian reversal of the
constants should wash out at compile time.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>