usb: gadget: zero: Add flexible auto remote wakeup test method

In order to increase test coverage, we can change the interval between
two remote wakeups every time, and the interval can be any user defined
value. This change will no affect current behavior if the user does not
use two introduced module paramters.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>