generic: add missing of.h header to ar8327

generic: add missing of.h header to ar8327

It seems new kernel linux version reorganized the header include and now
of.h needs to be explicitly included. This should have been done from
when the driver was introduced.

Add the missing of.h header to fix compilation error in later kernel
version.

Co-authored-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Signed-off-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/18455
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>