net: ppp: ppp_async: Fix data stall issue over low latency tty port.

Increase the receive room and avoid calling unthrottle
on a low latency tty port.

Change-Id: Idd4eb9f39d92e6c509423f55896922adaee9c224
Signed-off-by: Sridhar Ancha <sancha@codeaurora.org>