staging/fwserial: Limit tx/rx to 1394-2008 spec maximum

Per this conversation https://lkml.org/lkml/2012/11/27/587
limit the maximum transmission to the IEEE 1394-2008 specification
maximum size of 4096 bytes for asynchronous packets.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fwserial/TODO

@@ -12,9 +12,6 @@ TODOs
 1. This driver uses the same unregistered vendor id that the firewire core does
      (0xd00d1e). Perhaps this could be exposed as a define in
      firewire-constants.h?
-2. MAX_ASYNC_PAYLOAD needs to be publicly exposed by core/ohci
-   - otherwise how will this driver know the max size of address window to
-     open for one packet write?
 3. Maybe device_max_receive() and link_speed_to_max_payload() should be
      taken up by the firewire core?
 4. To avoid dropping rx data while still limiting the maximum buffering,

drivers/staging/fwserial/fwserial.c

@@ -179,7 +179,7 @@ static void dump_profile(struct seq_file *m, struct stats *stats)
 /* Returns the max receive packet size for the given card */
 static inline int device_max_receive(struct fw_device *fw_device)
 {
-	return 1 <<  (clamp_t(int, fw_device->max_rec, 8U, 13U) + 1);
+	return 1 <<  (clamp_t(int, fw_device->max_rec, 8U, 11U) + 1);
 }
 
 static void fwtty_log_tx_error(struct fwtty_port *port, int rcode)

drivers/staging/fwserial/fwserial.h

@@ -374,10 +374,10 @@ static inline void fwtty_bind_console(struct fwtty_port *port,
  */
 static inline int link_speed_to_max_payload(unsigned speed)
 {
-	static const int max_async[] = { 307, 614, 1229, 2458, 4916, 9832, };
-	BUILD_BUG_ON(ARRAY_SIZE(max_async) - 1 != SCODE_3200);
+	static const int max_async[] = { 307, 614, 1229, 2458, };
+	BUILD_BUG_ON(ARRAY_SIZE(max_async) - 1 != SCODE_800);
 
-	speed = clamp(speed, (unsigned) SCODE_100, (unsigned) SCODE_3200);
+	speed = clamp(speed, (unsigned) SCODE_100, (unsigned) SCODE_800);
 	if (limit_bw)
 		return max_async[speed];
 	else