ipq40xx: switch to 5.15 as default kernel
The testing kernel received now multiple months of testing. Set 5.15 as
default to give it a test with a broader audience.
Tested on:
- MikroTik SXTsq 5 AC
- FritzBox 4040/7530
- ZyXEL NBG6617
Signed-off-by: Nick Hainke <[email protected]>