Clear next_scan_freqs on wpa_supplicant FLUSH command
It was possible for old scan state to remain from a previous test case
when an operation like WNM neighbor scan or another-BSS-in-ESS was
started, but stopped at the end of a test case. This could result in
failures, e.g., when running wnm_bss_tm_req followed by scan_setband.
Signed-off-by: Jouni Malinen <j@w1.fi>