mwifiex: scan: Simplify code

This patch:
   - improves code layout
   - removes a useless memset(0) for some memory allocated with kzalloc
   - removes a useless if. We know that 'if (chan_band_tlv)' will succeed
     because it has been tested a few lines above

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>