usb: u_ether: Add missing rx_work init

commit 398a708e
usb: u_ether: Add workqueue as bottom half handler for rx data path

set up a worker for the rx data path but missed a case where the
work_struct needed to be initialized.

This patch adds the missing 'INIT_WORK'

Change-Id: I2daabd39d35b3e17a3054837282d649d9c78a0aa
Signed-off-by: Matthew Moeller <moeller.matt@gmail.com>