add 12.0gbps case

Reference: /n/atom/patch/applied2013/nixfis120
Date: Tue Nov 19 01:40:54 CET 2013
Signed-off-by: quansrto@quanstro.net

--- /sys/src/nix/port/sdfis.h	Tue Nov 19 01:40:44 2013
+++ /sys/src/nix/port/sdfis.h	Tue Nov 19 01:40:45 2013
@@ -7,6 +7,7 @@
 	Spd15		= 8,
 	Spd30		= 9,
 	Spd60		= 10,
+	Spd120		= 11,
 
 	SDspinup	= SDretry - 1,
 	SDrate		= SDretry - 2,