net: rmnet_usb_ctrl: Block poll() after device disconnect

Driver is setting POLLERR mask event upon device disconnect which
makes poll() non blocking call. This causes userspace client to
continuously poll for read after device disconnect. Hence clear
mask event next time when usespace client calls poll(). This allows
userspace client to get blocked until device connects back. Also
empty rx_list at the time of disconnect to avoid passing stale
data to userspace after device connects back.

CRs-Fixed: 641385
Change-Id: Iac6449c814e4a6016d3a4d5b4f7a27d9fd1dae6c
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>