rt-rpi-300.conf: Pin to kernel version 5.15

In order to use kernel 6.1 on this machine we'd need to rework some of the patches.

Signed-off-by: Florin Sarbu <florin@balena.io>

layers/meta-balena-raspberrypi/conf/machine/rt-rpi-300.conf

@@ -2,9 +2,8 @@
 #@NAME: Rocktech-RPI-300
 #@DESCRIPTION: Machine configuration for Rocktech-RPI-300
 
-MACHINEOVERRIDES = "raspberrypi4-64:${MACHINE}"
+MACHINEOVERRIDES =. "raspberrypi4-64:"
+
 include conf/machine/raspberrypi4-64.conf
 
-# because we override the raspberrypi4-64 machine which in turn is an override of raspberrypi4, we need to do the following gimmick:
-# courtesy of https://github.com/balena-os/balena-jetson/pull/112/commits/9d21df6899e595b4aeab4cc9a939ae6c564c669a
-MACHINEOVERRIDES := "${@'${MACHINEOVERRIDES}'.replace(':${MACHINE}',':raspberrypi4-64:${MACHINE}')}"
+PREFERRED_VERSION_linux-raspberrypi = "5.15.%"