bcm63xx: switch to 5.4 kernel

Seems stable after 6 days of testing on some of my devices.
Let's switch to 5.4 in order to get more feedback.
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

target/linux/bcm63xx/Makefile

@@ -12,8 +12,7 @@ BOARD:=bcm63xx
 BOARDNAME:=Broadcom BCM63xx
 SUBTARGETS:=generic smp
 FEATURES:=squashfs usb atm pci pcmcia usbgadget
-KERNEL_PATCHVER:=4.14
-KERNEL_TESTING_PATCHVER:=5.4
+KERNEL_PATCHVER:=5.4
 
 define Target/Description
 	Build firmware images for Broadcom based xDSL/routers