fix sign extension problem when writing to pciraw.
example from debugging, now removed:

writep(3, 8000 [0080], 2, 0x52)
devpnp: got16 ffffff80

Notes:
Wed Mar 16 14:44:52 EDT 2011 geoff
    already fixed.


Reference: /n/sources/patch/sorry/pcirawsignextend
Date: Wed Mar 16 15:58:50 CET 2011
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/src/9/port/devpnp.c	Wed Mar 16 15:58:45 2011
+++ /sys/src/9/port/devpnp.c	Wed Mar 16 15:58:44 2011
@@ -593,7 +593,7 @@
 	if(n >= sizeof(buf))
 		n = sizeof(buf)-1;
 	a = va;
-	strncpy(buf, va, n);
+	strncpy(buf, (char*)a, n);
 	buf[n] = 0;
 
 	switch(TYPE(c->qid)){