tests: Fix p2p_ext_vendor_elem_assoc test with P2P Device interface

tests: Fix p2p_ext_vendor_elem_assoc test with P2P Device interface

For configurations that use a dedicated P2P Device interface, which
mandates that a separate interface is used for the P2P group, vendor
specific IE's must be added to the VENDOR_ELEM_P2P_* frame types in
order to be used by the P2P group interface. The VENDOR_ELEM_ASSOC_REQ
(13) parameter would need to be issued on the group interface which
would be challenging to do due to timing in case a separate group
interface gets used.

In case a dedicated P2P Device interface is used, don't include a test
for VENDOR_ELEM_ASSOC_REQ to avoid failing this part of the test case.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>