usb: gadget: Fix bug in serial driver RX path

If low_latency flag is set tty_flip_buffer_push() API push the data
to line discipline in the same context.  When line discipline buffer
reaches above/below threshold throttle/unthrottle.  TTY use termios
mutext to serialize throttle/unthrottle methods.  Replace tasklet
with work queue so that TTY can acquire mutex.  A dedicated worker
thread is created for achieving good throughput.

Change-Id: I1ebbe1c9fee5591dace18940fb5c8716829f2dd2
Signed-off-by: Pavankumar Kondeti <pkondeti@qualcomm.com>