cpictl: Limit kernel sublevel to one byte

Linux stable kernels can reach greater than 256 sublevels [1],
which can cause the cpi tooling to generate an invalid string
that gets passed to the firmware and causes unusual responses:

  $ uname -r
  5.4.255
  $ cat /sys/firmware/cpi/system_level
  0x04260000000504ff

  --reboot--

  $ uname -r
  5.4.256
  $ cat /sys/firmware/cpi/system_level
  0x4260000000504100

The first sublevel field is defined as one byte, so ensure that
a value larger than that isn't included.

[1] https://lore.kernel.org/lkml/1612534196241236@kroah.com/

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

scripts/cpictl

@@ -246,6 +246,9 @@ get_system_level()
 	read -r k_ver k_patchlvl k_sublvl k_sublvl2 <<< "$(split_kver "$kver_str" 4)"
 
 	# Handle excessive sublevel numbers consistently
+	if [[ "$k_sublvl" -gt 255 ]] ; then
+		k_sublvl=0
+	fi
 	if [[ "$k_sublvl2" -gt 65535 ]] ; then
 		k_sublvl2=0
 	fi