usb: xhci: fix timeout for transition from RExit to U0

This definition is used by msecs_to_jiffies in milliseconds.
According to the comments, max rexit timeout should be 20ms.
Align with the comments to properly calculate the delay.

Verified on Sunrise Point-LP and Cannon Lake.

Cc: stable@vger.kernel.org
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>