tty: n_smux: Adjust receive retry queue parameters

If the remote modem has queued many small packets in the UART FIFO and
the SMUX client cannot provide receive buffers fast enough, then the
auto flow control command may not get processed before the receive queue
has overflowed.  In measured cases, an average of 10 packets are
received after flow control is enabled.  In the failure cases, up to 19
packets are received which is more than the max queue size (32 packets)
minus the high watermark (16 packets) which causes an overflow.

Use a bigger retry queue and more aggressive watermark values to make
auto flow control more responsive and to allow more packets to get
queued before an overflow occurs.

CRs-Fixed: 394198
Change-Id: I53f564a5401ea60f351e2e6b29ec41eb4b806653
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>