usb: xhci: Add helper function xhci_set_power_on().

Refactoring port power on/off related code into
a helper function xhci_set_power_on() which can
be reused when enabling test mode.

[set port state to neutral before writing port power -Mathias]
Signed-off-by: Guoqing Zhang <guoqing.zhang@intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>