USB: xhci: Refactor code to clear port change bits.

Refactor the code to clear the port change bits in the port status
register.  All port status change bits are write one to clear.

Remove a redundant port status read that was supposed to unblock any
posted writes.  We read the port after the write to get the updated status
for debugging, so the port read after that is unnecessary.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>