sifiveu: use kernel 6.12 by default

Switch to Linux kernel version 6.12.

Signed-off-by: Zoltan HERPAI <[email protected]>

target/linux/sifiveu/Makefile

@@ -11,8 +11,7 @@ FEATURES:=ext4
 KERNELNAME:=Image dtbs
 SUBTARGETS:=generic
 
-KERNEL_PATCHVER:=6.6
-KERNEL_TESTING_PATCHVER:=6.12
+KERNEL_PATCHVER:=6.12
 
 include $(INCLUDE_DIR)/target.mk