USB: serial: digi_acceleport: fix OOB-event processing

A recent change claimed to fix an off-by-one error in the OOB-port
completion handler, but instead introduced such an error. This could
specifically led to modem-status changes going unnoticed, effectively
breaking TIOCMGET.

Note that the offending commit fixes a loop-condition underflow and is
marked for stable, but should not be backported without this fix.
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: 2d380889 ("USB: serial: digi_acceleport: fix OOB data sanity
check")
Cc: stable <stable@vger.kernel.org>	# v2.6.30: 2d380889Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/digi_acceleport.c

@@ -1500,7 +1500,7 @@ static int digi_read_oob_callback(struct urb *urb)
 		return -1;
 
 	/* handle each oob command */
-	for (i = 0; i < urb->actual_length - 4; i += 4) {
+	for (i = 0; i < urb->actual_length - 3; i += 4) {
 		opcode = buf[i];
 		line = buf[i + 1];
 		status = buf[i + 2];