Fix remaining incorrect reference to target/at91sam3uXX.cfg

Commit 1794e5ee54 renamed the file to
have all lowercase characters according to most references to the
file, but the commit didn't change the existing reference to the
old filename.

Change-Id: I380e52e947a8091d48cf010e3919bf2caed7fdff
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://openocd.zylin.com/248
Tested-by: jenkins

tcl/target/at91sam3u4e.cfg

@@ -1,5 +1,5 @@
 # common stuff
-source [find target/at91sam3uXX.cfg]
+source [find target/at91sam3uxx.cfg]
 
 # size is automatically "calculated" by probing
 set _FLASHNAME $_CHIPNAME.flash0