hla: add ability to configure read/write buffer size

Other adapters (TI ICDI) that use this driver can use a larger
read/write buffer size than the original stlink could.

Change-Id: I9beb7748049097cbe29a2340799c450bd74e199d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/948
Tested-by: jenkins

src/jtag/drivers/stlink_usb.c

@@ -1160,6 +1160,9 @@ static int stlink_usb_open(struct hl_interface_param_s *param, void **fd)
 
 	h->transport = param->transport;
 
+	/* set max read/write buffer size in bytes */
+	param->max_buffer = 512;
+
 	const uint16_t vids[] = { param->vid, 0 };
 	const uint16_t pids[] = { param->pid, 0 };