usb: core: add a new usb_get_ptm_status() helper

Drivers who are interested in the PTM status stype, should use this
new helper to make sure they issue the correct GetStatus message.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>