usb: serial: Stop pushing rx data when tty node is removed

usb_wwan driver continues pushing rx data buffer to tty node
even if it is removed during device disconnect. This may cause
watchdog bark as driver unnecessarily loops through the list of
rx buffers and tries to push it to tty layer while holding
spin lock with interrupts disabled.

CRs-Fixed: 377066
Change-Id: I5569a4948aba89fd1d0d8ea5e8910ce2e0c8e917
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>