ath25: switch default kernel to 5.15

Switch to kernel 5.15 although it is marked as source only. It is marked
as source only because nobody was able to test this device.

Merging this is important to finally branch a new release. We want to
get rid completely of 5.10 kernel. Without this commit we get following
error: "Missing kernel version/hash file for 5.10." Hopfully someone
will fix the board in the rc phase or it will be removed completly.

Signed-off-by: Nick Hainke <[email protected]>

target/linux/ath25/Makefile

@@ -10,8 +10,7 @@ BOARDNAME:=Atheros AR231x/AR5312
 FEATURES:=squashfs low_mem small_flash source-only
 SUBTARGETS:=generic
 
-KERNEL_PATCHVER:=5.10
-KERNEL_TESTING_PATCHVER:=5.15
+KERNEL_PATCHVER:=5.15
 
 define Target/Description
 	Build firmware images for Atheros SoC boards