tests: Reset the correct key's PN for group key handshake testing

tests: Reset the correct key's PN for group key handshake testing

While adding support for IGTK and BIGTK here, I tested this without
protections (i.e., with protections removed from both wpa_supplicant and
the driver), and while I got some bad resets on the debugfs values, it
should have failed with "unexpected connectivity".

Fix this to be correct - we need to reset the GTK PN, not the PTK PN in
this test.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>