wpa-supplicant: Match package override to PACKAGES for pkg_postinst

In PACKAGES, ${PN} is used so it makes sense for the pkg_postinst variable
override to match that else it causes user confusion.

[YOCTO #14616]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ae9094d4)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>