cfg80211: fix alignment problem in scan request

The memory layout for scan requests was rather wrong,
we put the scan SSIDs before the channels which could
lead to the channel pointers being unaligned in memory.
It turns out that using a pointer to the channel array
isn't necessary anyway since we can embed a zero-length
array into the struct.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>