wl12xx/wl18xx: scan all 5ghz channels

Due to a typo, the current code copies only sizeof(cmd->channels_2)
bytes, which is smaller than the correct sizeof(cmd->channels_5)
size, resulting in a partial scan (some channels are skipped).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>