qtnfmac: check band before allocating cmd_skb to avoid resource leak

The current code allocates cmd_skb and then will leak this if band->band
is an illegal value. It is simpler to sanity check the band first before
allocating cmd_skb so that we don't have to free cmd_skb if an invalid
band occurs.

Detected by CoverityScan, CID#1437561 ("Resource Leak")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>