bcm63xx: use kernel 5.4 as testing kernel
This uses 5.4 as testing kernel. Since 4.19 has not seen broad
testing yet, just keep 4.14 as stable kernel until 5.4 is ready.
Tested on Comtrend AR-5387un (Thanks to @Noltari).
Signed-off-by: Adrian Schmutzler <[email protected]>