usb: gadget: rndis: add flexibility to trigger RX via SysFS

Currently RNDIS/uether driver starts when usb cable is connected
and network interface is up. This might lead to DHCP pkt drop (very
initial packets from host/windows PC) if RNDIS bridges are not setup
appropriately in userspace. DHCP packet drop results in delayed IP
assignment.
Provide SysFS parameter to enable and trigger RX from
userspace.

Change-Id: I9dbef11eb97ac87bff47cea1138fe5d7e343377f
Signed-off-by: Amit Blay <ablay@codeaurora.org>