change confusing error message that looks like and endpoint #, not bytes

Reference: /n/atom/patch/applied/usbsaybytes
Date: Tue Mar 11 09:52:10 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/usb/serial/serial.c	Tue Mar 11 09:51:53 2014
+++ /sys/src/cmd/usb/serial/serial.c	Tue Mar 11 09:51:54 2014
@@ -464,7 +464,7 @@
 }
 
 enum {
-	Serbufsize	= 255,
+	Serbufsize	= 256,
 };
 
 static long
@@ -568,7 +568,7 @@
 
 	ser = p->s;
 	do{
-		dsprint(2, "serial: write to bulk %ld\n", count);
+		dsprint(2, "serial: write %ld bytes to bulk\n", count);
 
 		if(ser->wait4write != nil)
 			/* unlocked inside later */