net: usb: rmnet_usb_ctrl: Honor device polling interval

Driver is not using polling interval from device end point
descriptor. Instead driver is using hard coded value which
is inconsistent with the polling frequency required by the
endpoint of the device. If device requires polling interval
shorter than hard coded value it leads to latency to the
end to end communication. Hence use polling interval from
endpoint descriptor of attached device.

Change-Id: Iefa268ada9c672b5ccd805030b0f813f1ad9f286
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>