usb: u_ether: Prevent dropping multicast packet in rmnet ip mode

In rmnet ip mode ftp login response is getting dropped quietly by
ndo_start_xmit call back. Fix this by checking rmnet ip mode in
ndo_start_xmit call back. Also increment drop counter to be able
to track the packet drop.

Change-Id: I11ba23673bbd162bea302f8bb1a930c58e84b182
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>