treewide: replace /sys/devices/virtual/ubi by /sys/class/ubi

treewide: replace /sys/devices/virtual/ubi by /sys/class/ubi

Starting from Linux Kernel version 6.3 UBI devices will no longer be
considered virtual, but rather have an MTD device parent. Hence they
will no longer be listed under /sys/devices/virtual/ubi which is
used in multiple places in OpenWrt. Prepare for future kernels by
using /sys/class/ubi instead of /sys/devuces/virtual/ubi.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>