USB: u_serial: Don't push data to line discipline in interrupt context

Currently setting low_latency flag in u_serial driver which causes
pushing data from tty layer to line discipline in interrupt contect.
As part of this, calling sleeping function from atomic context which
could crash. Fix this by not setting low_latency flag.

CRs-Fixed: 393614
Change-Id: Id79b8972cfa823c173de158e0ccf08a6d052057f
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>