supplicant:  Assume external scans complete within 6 seconds.

When testing relatively buggy ath10k driver and firmware,
I saw cases where supplicant thought external scans were
active when in fact none were.  This effectively hangs supplicant
because it will never again scan to start the association logic.

This patch times out external scans after 6 seconds and continues
on as if the external scan had completed.

Signed-off-by: Ben Greear <greearb@candelatech.com>