USB: serial: keyspan_pda: increase transmitter threshold

Increase the transmitter threshold so that writing isn't resumed until
128 bytes are available in the device buffer thereby allowing for larger
and more efficient transfers.

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/keyspan_pda.c

@@ -40,7 +40,7 @@
 #define DRIVER_AUTHOR "Brian Warner <warner@lothar.com>"
 #define DRIVER_DESC "USB Keyspan PDA Converter driver"
 
-#define KEYSPAN_TX_THRESHOLD	16
+#define KEYSPAN_TX_THRESHOLD	128
 
 struct keyspan_pda_private {
 	int			tx_room;